PURPOSE: To obtain a high attenuation without using a multi-stage by providing a prescribed transmission line from an input terminal to an output terminal in addition to an attenuation path comprising a diode.

CONSTITUTION: The transmission line 21 is coupled at the input terminal side K1 and the output terminal K2. The degree of coupling at the points K1 and K2 is selected to be nearly equal to a maximum attenuation from the point K1 to the point K2 through a strip line 11, a signal attenuation diode 4 and a strip line 12. Thus, an attenuated signal via the strip line 12 and a signal via the transmission line 21 are cancelled when the signals are synthesized at the point K2. The length of the transmission line 21 is selected so as to make both the signals opposite in phase for the cancelling. Thus, a high attenuation is obtained without multi-stage provision.