The home side managed to cross the whitewash on three occasions including a try from full international winger Dafydd Howells.
The match-day squad included six young players from the Cardiff Blues. Scrum-half Tom Williams and prop Callum Lewis proved vital in the win.
The team will face Ireland at Dubarry Park, Athlone, next Friday (19.35 KO)
Wales Under-20: Afon Bagshaw (RGC); Dafydd Howells (Ospreys), Harri Evans (RGC), Steffan Hughes (capt, Scarlets), Joshua Adams (Scarlets); Ethan Davies (Dragons), Tom Williams (Blues); Callum Lewis (Blues), Elliot Dee (Dragons), Nicky Thomas (Ospreys ), Ben Roach (Blues), James Sheekey (Blues), Will Boyde (Carmarthen), Scott Matthews (Dragons), James Benjamin (Dragons).
Replacements: Scott Otten (Ospreys), Leon Crump (Blues), Benjamin Leung (Scarlets), Scott Andrews (Dragons), Connor Lloyd (Scarlets), Luke Price (Ospreys), Garyn Smith (Blues).