PURPOSE: To raise the output of a microwave high output amplifier by providing a strip along the propagation direction of microwaves on a strip line and physically and approximately equalizing a route length from an input terminal to the center part or both end parts of a semiconductor amplifier element or the like.

CONSTITUTION: Microwave signals made incident from the input terminal 4 are passed through an input matching circuit 5 composed of a microstrip line 2, supplied to the center part and both end parts of an FET 1 in common-mode and respectively amplified. Further, the amplified respective microwave signals are synthesized in the common-mode through an output matching circuit 8 composed of the microstrip line 6 and supplied through an output terminal 7 to the loads of an antenna or the like. In this amplifier, by providing the slits 10 and 11 respectively on the microstrip lines 2 and 6 for constituting the input matching circuit 5 and the output matching circuit 8, the route lengths of the microwaves from the input terminal 4 to the output terminal 7 passing through the center part and both end parts of the FET 1 are approximately equalized and the output of the amplifier is increased.