PURPOSE: To effectively utilize the plasma gas produced in a reaction vessel for the formation of a film by providing microwave cutoff cavities having a cutoff frequency different from each other on opposite sides of a waveguide.

CONSTITUTION: The microwave is transmitted from a power source 101 through a matching device 103 and the waveguide 104, raw gases are introduced into the reactor 107 from cylinders 118W120 through gas inlets 121 and 109 to produce plasma, and a deposited film is formed on a substrate 110. At the time, the first and second microwave cutoff cavities 105 and 106 having a cutoff frequency different from each other are provided on opposite sides of the waveguide 104 on the electric field or vertical surface in the vicinity of the terminal of the waveguide 104. The cavities 105 and 106 are formed by a metallic tube opened on both ends, and the cutoff frequency f1 of the first cavity 105 is made lower than the cutoff frequency f2 of the second cavity 106. As a result, the microwave energy does not leak into the first cavity 105, and the active species can be efficiently formed.