PURPOSE: To suppress the deterioration of the working efficiency due to the control of a strip circuit, etc. and also to shorten the time for production of an alteration pattern, by using an element whose resistance value varies electrically to the termination resistance of a hybrid ring circuit and therefore obtaining a filter having variable Q.

CONSTITUTION: A hybrid ring circuit consists of an input terminal 2 and output terminals 3 and 4 together with strip lines (1/4 wavelength) 1a∼1d connected in a ring form. An element whose resistance value varies electrically, such as a PIN diode 6 is used to the termination resistance of said ring circuit. The drive circuits 7a∼7c are added to the circuit, and an output is obtained through the terminal 3.