Blues players Leigh Halfpenny, Scott Andrews, Jevon Groves and Sam Warburton all keep their places, while full back Daniel Evans (ankle), lock Haydn Pugh (ankle) and flanker Dan Franks (shoulder) are nursing injuries but should be in contention for Saturday's showdown against France.
Blues Academy Centre Luke Ford will earn his first Under 20 cap after suffering a broken finger ahead of the Six Nations campaign, while fellow Academy colleague, Patrick Palmer (Prop), injured playing on loan for the Dragons at the end of the Magners League season, has returned to full fitness to take up a place on the bench.
Horgan said: "Some of the players coming in, such as Justin Tipuric and Lloyd Phillips, performed well for us throughout the Six Nations, while others such as Jason Tovey for the Dragons and Nathan Rees for Cardiff deserve their chance after impressing for their club or region.
"There is no reason for these players not to perform - all 26 players in the squad are capable of earning their place in the side to face France."
Horgan is looking for more accuracy in his side's performance tomorrow night after missing chances against the Azzurri.
"We had a number of missed try-scoring opportunities against Italy so we have been working on our execution and accuracy ahead of the Japan game.
"However, Italy's defence was excellent and certainly kept them in the game.
"You are always looking for improvement but we are quite pleased overall with our first performance. Japan gave a good account of themselves against France and we will certainly give the side the respect they deserve tomorrow night.
"The crowd supported us well on Friday night and we hope we can give them something more to shout about on Tuesday."
Wales play Japan at the Liberty Stadium, Swansea at 7.10pm on Tuesday, 10 June, while Pool D rivals France will open the evening's rugby action against Italy at 5.00pm, also at the Liberty Stadium.
Elsewhere on Tuesday, in Pool A, tournament top seed New Zealand will play Ireland at 9.10pm at the Arms Park after Tonga take on Argentina at 7.00pm.
Tickets for all Junior World Championship match days are £2 for children and £8 for adults and are available at all match venues right up until kick-off.
Wales Under 20 team v Japan: 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 (Capt, Blues)
Replacements: Rhys Lawrence (Ospreys), Ryan Bevington (Ospreys), Patrick Palmer (Blues), Josh Turnbull (Scarlets), Jonathan Davies (Scarlets), Rhys Webb (Ospreys), Gareth Owen (Ospreys)