A plasma treating device which can generate uniform plasma over a wide area and can perform plasma treatment on a substrate (wafer W) to be treated with high in-plane uniformity. A transmissive window (23) for transmitting microwaves is provided in the ceiling section of a vacuum vessel (2) provided with a plasma chamber (21) and a film forming chamber (22) and a waveguide (4) for supplying microwaves of 2.45 GHz in frequency to the vacuum vessel (2) in a TM mode is provided on the window (23). The waveguide (4) is composed of a rectangular waveguide (41), a cylindrical waveguide (42) which works as a TM mode converter (42), and a conical waveguide (43) and the exit side of the waveguide (43) is connected to the upper surface of the window (23). When the microwaves are transmitted into the vessel (2) in the TM mode and, at the same time, a magnetic field is formed in the vessel (2) and the exit-side inside diameter ØA of the waveguide (43) is adjusted to 130-160 mm, the density of plasma in the plasma chamber (1) becomes uniform and plasma treatment can be performed on a wafer W having a size of, for example, 8 inches with high in-plane uniformity.