Despite the try rout for Wales, it was Japan who kicked off the scoring with an early penalty from full back Noguchi.
However, once Blues outside half Jarrod Evans had converted his own try, in the sixth minute, Wales were rarely troubled. Blues wing Benham, playing for the first time in the tournament, crossed for the first of his two tries, four minutes later, and Scarlets Tom Phillips, at No 8, added a third in the 19th.
With Evans converting and Japan down to 14 men following a yellow card to openside flanker Urabe, Wales added a fourth try, once again, from the lively right wing Benham.
On the stroke of half time, it was 33-3 when a Japanese turnover enabled Blues scrum half Tomos Williams and Dragons flanker Ollie Griffiths to combine and send in Adams for a smart try that Evans failed to convert, from the touchline.
With the game safely won, it was all about creating and finishing opportunities, after the break, something that had been lacking in Wales' first two games of the tournament against France and England. Ospreys lock Adam Beard scored a sixth - Evans converting - fellow Osprey Dafydd Howells a seventh and Blues hooker Liam Belcher an eighth after a wonderful catch and sideways kick from Benham. Replacement outside half Daniel Jones converted.
Seb Davies touched down at the posts for Scarlet pivot Jones to convert again, eight minutes from time and with three minutes left, Adams took a scoring pass from Howells for try number 10.
Captain Rory Thornton said: "It was a massive win for us. The focus was to start well because we let ourselves down in the first 20 minutes of the first two games. This time, we got the points and we got the start we wanted.
"It has been a bit difficult because we have been dropping balls. But that happens and you just have to move on and do what you are doing. It's a good win."
Coach Geraint Lewis added: "We are pleased with the win and the performance. We moved the ball well and we scored some good tries. Hopefully we will carry that into the rest of the tournament.
"We started well, got some momentum and we made it hard for Japan to get back into the game.
"We played two good sides in England and France, but the boys have taken a lot of confidence from today's game and hopefully we can kick on from here."
Wales Under 20 v Japan:
Dafydd Howells (Ospreys/ Neath); Elis-Wyn Benham (Blues/ Cardiff), Garyn Smith (Blues/ Pontypridd), Owen Watkin (Ospreys/ Bridgend), Joshua Adams (Scarlets/ Llanelli); Jarrod Evans (Blues/ Pontypridd), Tomos Williams (Blues/ Pontypridd); Luke Garrett (Dragons/ Cross Keys), Ryan Elias (Scarlets/ Carmarthen), Dillon Lewis (Blues/ Pontypridd), Adam Beard (Ospreys/ Aberavon), Rory Thornton (captain, Ospreys/ Bridgend), Seb Davies (Blues/ Pontypridd), Ollie Griffiths (Dragons/ Newport), Tom Phillips (Scarlets/ Llanelli)
Replacements: Torin Myhill (Scarlets/ Llanelli), Keagan Bale (Dragons/ Cross Keys), Joe Jones (Blues/ Cardiff), Lee Taylor (Carmarthen Quins), Liam Belcher (Blues/ Pontypridd), Kieran Hardy (Scarlets/ Carmarthen), Daniel Jones (Scarlets/ Carmarthen), Joe Thomas (Ospreys/ Swansea)
However, once Blues outside half Jarrod Evans had converted his own try, in the sixth minute, Wales were rarely troubled. Blues wing Benham, playing for the first time in the tournament, crossed for the first of his two tries, four minutes later, and Scarlets Tom Phillips, at No 8, added a third in the 19th.
With Evans converting and Japan down to 14 men following a yellow card to openside flanker Urabe, Wales added a fourth try, once again, from the lively right wing Benham.
On the stroke of half time, it was 33-3 when a Japanese turnover enabled Blues scrum half Tomos Williams and Dragons flanker Ollie Griffiths to combine and send in Adams for a smart try that Evans failed to convert, from the touchline.
With the game safely won, it was all about creating and finishing opportunities, after the break, something that had been lacking in Wales' first two games of the tournament against France and England. Ospreys lock Adam Beard scored a sixth - Evans converting - fellow Osprey Dafydd Howells a seventh and Blues hooker Liam Belcher an eighth after a wonderful catch and sideways kick from Benham. Replacement outside half Daniel Jones converted.
Seb Davies touched down at the posts for Scarlet pivot Jones to convert again, eight minutes from time and with three minutes left, Adams took a scoring pass from Howells for try number 10.
Captain Rory Thornton said: "It was a massive win for us. The focus was to start well because we let ourselves down in the first 20 minutes of the first two games. This time, we got the points and we got the start we wanted.
"It has been a bit difficult because we have been dropping balls. But that happens and you just have to move on and do what you are doing. It's a good win."
Coach Geraint Lewis added: "We are pleased with the win and the performance. We moved the ball well and we scored some good tries. Hopefully we will carry that into the rest of the tournament.
"We started well, got some momentum and we made it hard for Japan to get back into the game.
"We played two good sides in England and France, but the boys have taken a lot of confidence from today's game and hopefully we can kick on from here."
Wales Under 20 v Japan:
Dafydd Howells (Ospreys/ Neath); Elis-Wyn Benham (Blues/ Cardiff), Garyn Smith (Blues/ Pontypridd), Owen Watkin (Ospreys/ Bridgend), Joshua Adams (Scarlets/ Llanelli); Jarrod Evans (Blues/ Pontypridd), Tomos Williams (Blues/ Pontypridd); Luke Garrett (Dragons/ Cross Keys), Ryan Elias (Scarlets/ Carmarthen), Dillon Lewis (Blues/ Pontypridd), Adam Beard (Ospreys/ Aberavon), Rory Thornton (captain, Ospreys/ Bridgend), Seb Davies (Blues/ Pontypridd), Ollie Griffiths (Dragons/ Newport), Tom Phillips (Scarlets/ Llanelli)
Replacements: Torin Myhill (Scarlets/ Llanelli), Keagan Bale (Dragons/ Cross Keys), Joe Jones (Blues/ Cardiff), Lee Taylor (Carmarthen Quins), Liam Belcher (Blues/ Pontypridd), Kieran Hardy (Scarlets/ Carmarthen), Daniel Jones (Scarlets/ Carmarthen), Joe Thomas (Ospreys/ Swansea)
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