Wales U20 (26) 33
Tries: Tovey, Rees (2), Ford, Halfpenny Cons:
Japan U20 (0) 10
Tries: Havea, Mikami
Wales Under-20 set up an IRB Junior World Championship Pool D decider with France on Saturday after labouring to a bonus-point win over Japan in Swansea.
Jason Tovey, Nathan Rees, Luke Ford and Leigh Halfpenny got first-half tries, but indiscipline saw Japan fight back.
Aisea Havea and Masataka Mikami got tries as Sam Warburton, Sam Hobbs and Eiri Nakada were sin-binned, before Rees' second try finished the scoring.
Earlier at the Liberty Stadium, France got a 32-14 bonus-point win over Italy.
With both Les Bleus and Patrick Horgan's Wales side also having claimed bonus-point wins in their opening games, the group will be up for grabs in Saturday's showdown in Swansea.
"We will be confident going into the France game," said man-of-the-match Halfpenny.
"Japan were a tough team to break down, but we stuck to our structure and plan.
"It was a quiet second half without much ball, but personally I was very pleased with my performance."
Wales Under-20: Jason Tovey (Dragons); Leigh Halfpenny (Blues), Luke Ford (Blues), Rhys Williams (Scarlets), Nathan Rees (Blues); Dan Biggar (Ospreys), Gareth Williams (Ospreys); Sam Hobbs (Blues), Ryan Prosser (Bristol), Scott Andrews (Blues), Ashley Sweet (Dragons), Jevon Groves (Blues), Lloyd Phillips (Scarlets), Justin Tipuric (Ospreys), Sam Warburton (Blues, capt).
Replacements: Rhys Lawrence (Ospreys), Ryan Bevington (Ospreys), Patrick Palmer (Blues), Josh Turnbull (Scarlets), Jonathan Davies (Scarlets), Rhys Webb (Ospreys), Gareth Owen (Ospreys).
Referee: Peter Fitzgibbon (Ireland)