PURPOSE: To attain the variable attenuation while keeping a constant impedance, by controlling a current flowed to a series circuit consisting of diodes and resistors provided at input and output terminals of a 1/4 wavelength transmission line.

CONSTITUTION: An output signal from a signal source 2 is transmitted to a 1/4 wavelength transmission line 10 via a coupling capacitor C1 and outputted from a terminal 5 via a coupling capacitor C2. In applying a DC control voltage to a terminal 6, a current flows to diodes D1, D2. The high frequency resistance of the diodes is changed depending on the current value. Thus, the attenuation of a signal transmitted from the signal source 2 can be changed. The resistance value of resistors R1, R2 connected in series with the diodes D1, D2 is made equal to the characteristic impedance of the line 10, allowing to make the impedance viewed from the terminal 5 constant.