Disclosed is improvement of a quartz-glass top plate to be used as a microwave-transmitting window in a microwave plasma processing apparatus. The surface, facing a substrate W, of the top plate has surface roughness equal to or less than 0.2 μm in arithmetic mean surface roughness Ra. Thereby, generation of particles derived from the quartz glass material constituting the top plate is minimized, even when the top plate is exposed to a severe environment of high electron density and high electron temperature.