A microwave plasma processing method and, in which a linear plasma is produced by means of a microwave, and an object to be processed is subjected to processing under atmospheric pressure or under a pressure near atmospheric pressure when the object is moved, while a surface of the object is maintained at a horizontal position with respect to the linear plasma. A plasma head has an H-plane slot antenna, and slots are arranged alternately on both sides of a centerline of a waveguide at a pitch of λg/2 (λg: wavelength of the microwave with the waveguide). A uniforming line having a distance of n·λg/2 from the slots to an emission end of the plasma head is provided (n: an integral number).