A plasma processing device comprising a chamber (1) for accommodating therein a substrate (11), a high-frequency power supply (5) for generating microwave, and an antenna unit (3) for radiating microwave into the chamber (1). Microwave generated in the power supply (5) is sent to the antenna unit (3) via a waveguide (6). A top plate (4) forming part of a partition wall of the chamber (1) is formed at the upper portion of the chamber (1). A specified annular delay pass unit (2) formed of the same material as that of the top plate (4), for delaying the propagation of microwave, is provided on the outer peripheral portion of the top plate (4). Accordingly, the plasma processing device can restrict an abnormal discharge and the production of the foreign matters.