Wales kept the error count low and dominated possession against a Japanese team which had little answer to the all-round power of the home side. The Welsh handling proved a major weapon in attack with forwards and backs combining at will to find holes in a weakening opposition defence.
The result is bound to prove a boost to Welsh preparations for the forthcoming Junior World Championship in South Africa.
Wales started the try fest after just five minutes when Bridgend and Ospreys wing Luke Morgan crossed for the first score.
Throughout the first half Japan saw little of the ball as Wales notched up an eight try haul. Apart from the relentless passing game Wales overpowered the visitors in the scrum to add to their overall dominance. Japan refused to give up and grabbed a consolation first half try on 40 minutes when wing Kentaro Kawahara crossed.
Wales senior squad member Harry Robinson showed plenty of pace out wide but did not receive a scoring pass throughout the half, while four-times Wales capped Tom Prydie crossed for five tries to show his pace and ability in his first outing at U20 level.
Wales broke through the century barrier when replacement centre Thomas Pascoe ran 40 metres to touch down under the posts.
Coach Danny Wilson said: "I am delighted by the way we stuck to our game plan and kept our shape. It was a clinical performance with some great tries which proved the level of potency we have in our backs.
"To be fair to Japan they never gave up and provided us with the sort of workout we needed leading into a tough Junior World Championship group."