This microwave plasma processing apparatus has, as a gas introduction mechanism for introducing a working gas inside a chamber (10), electrical discharge prevention members (96(1) to 96(8)), each of which is provided to a plurality of dielectric window gas passages (94(1) to (94(8)) through which a dielectric window (54) passes. Each electrical discharge prevention member (96(n)), a portion (114) of which protrudes only a height h, which is greater than or equal to a predetermined distance H, upward from the rear surface of a dielectric window (52) on the inlet side, passes through an opening (54a) of a slot plate (54), and inserts into a branched gas supply path (92(n)) of a gas branch part (90). The gas branch part (90), spring coils (116) and the slot plate (54), which surround the protruding portion (114) of each electrical discharge prevention member (96(n)), constitute an enclosing conductor (118).