To suppress the output of spurious frequencies except an object frequency band of the microwave amplifier over a wide band.

One terminal of a chip resistor 5 is connected in parallel with an output terminal 2 at an output of the microwave amplifier formed on a dielectric board via a matching circuit 4 and an open stub 6 whose width is made wider toward an outer ridge from a base is connected to the other terminal of the chip resistor 5. The length in average from the base to the outer ridge of a sectorial open stub 6 is nearly 1/4 of wavelength of an spurious frequency band on the board and nearly 1/10 of an object frequency band on the board or below.