A plasma reactor for diamond synthesis includes a microwave generator, a waveguide connected to the microwave generator, an antenna disposed within the waveguide to direct the microwaves propagated along the waveguide toward the interior of a reaction chamber, a microwave window provided above the upper wall of the waveguide, a reaction chamber defined by (a) a cylindrical bottom member hermetically joined to the microwave window and the waveguide, (b) a reaction gas inlet port and a gas outlet port in the side wall thereof, and (c) a substrate holder disposed within the reaction chamber in facing opposition to the microwave window so as to be moved toward and away from the microwave window to adjust the distance between the microwave window and the substrate holder to generate a desired microwave resonance mode. A plasma is produced only in the central portion of the reaction chamber, so that the etching of the microwave window and the resulting contamination of the diamond film by impurities produced by etching the microwave window are prevented. The plasma reactor for diamond synthesis is capable of forming a high-quality diamond film on a large surface of a substrate at a high growth rate in a range of 1 to 2 .mu.m/hr.