PURPOSE: To absorb a reflected wave without using an isolator.

CONSTITUTION: A PIN diode 12 is connected between a strip line 11 and the earth. A PIN diode 13 is connected to the position, which is distant from the connection point of the PIN diode 12 to the input end side by odd-fold 1/4 wavelength, on the strip line. A resistance 14 having the same resistance value as the characteristic impedance of the strip line is connected in series to the PIN diode 13. When a diode switch is turned off, a forward bias voltage is simultaneously applied from a bias supply circuit 15 to PIN diodes 12 and 13. The micro wave signal propagated in the strip line is reflected by the PIN diode 12 and is absorbed through the PIN diode 13 by the resistance 14.