PURPOSE: To compose a circuit having a stable characteristic in a microwave band even at a higher degree, and to make variable an amplitude compensating quantity in the microwave band by cascade connecting strip line couplers in multiple stages, and providing a PIN diode and its bias circuit.

CONSTITUTION: Strip line 1/4 wavelength couplers 11, 12,...1n are coupled in an (n) stage, and a PIN diode 2, a choke coil 3, a direct current preventing capacitor 4, and a variable resistor are linked as shown in a figure. Thus, when the coupling lines 11, 12,-...1n at the 1/4 wavelength is cascade connected in the multiple stages, an equalizer at the higher degree can be composed, and when the circuit to apply the degree of the equalizer is composed of the coupling lines 11, 12,...1n at the 1/4 wavelength, the variable amplitude equalizer having the stable characteristic in the microwave band can be realized.