PURPOSE: To perform impedance matching to obtain a maximum gain by providing an interdigital coupling line and an adjusting mechanism in an input/output matching circuit connected to a three-terminal microwave semiconductor element.

CONSTITUTION: An input/output matching line 5 and a signal transmission line 4 connected to a three-terminal microwave semiconductor element 1 are provided on a dielectric substrate 2. An interdigital coupling line 8 is connected to this transmission line. A dielectric chip 9 having a small dielectric loss and a high dielectric constant is stuck onto the line 8 by an adhesive 10. Thus, the impedance of the line 8 can be varied. Consequently, the impedance is adjusted and is matched to obtain a maximum gain even in case of variance of the bonding position of the semiconductor element 1 or variance of the element 1 itself.