A microwave plasma processing apparatus performs plasma processing on a substrate by exciting a gas by electric field energy of microwaves emitted from a radial line slot antenna (RLSA). The microwave plasma processing apparatus includes: a processing container in which plasma processing is performed; a microwave source outputting microwaves; a rectangular waveguide transmitting the microwaves outputted from the microwave source; a coaxial converter converting a mode of the microwaves transmitted to the rectangular waveguide; a coaxial waveguide including an inner conductor slidably and electrically connected to the coaxial converter by a first contact member; the first contact member fixed to the coaxial converter and slidably contacting the inner conductor; and a first spring member absorbing displacement, which is caused by thermal expansion, of the RLSA and a member disposed above the RLSA.