PURPOSE: To reduce expansion of the spectrum by compounding the 0 and π phase modulation signals of the ring modulator and the output carrier signal of the switching circuit with the phase shift by π/2 and thus reducing the sudden change of the amplitude properties at the phase displacement point.

CONSTITUTION: Carrier wave signal 7 is turned into the value featuring the different phase by 90° through 90° hybrid 10 and then divided into two parts. And one of the two parts is supplied to ring modulator I, and the other is supplied to switching circuit II. Under these conditions, base band pulse signal 3 is applied to mudulator I via LPF4 and also applied to diode 8 of circuit II with the waveform as it is. And output B of modulator I is compounded with output C of circuit II which had a high-speed operation through compounding unit 11, thus delivering the phase modulation signal featuring the reduced expansion of the spectrum. In such constitution, the sudden change is reduced for the amplitude properties at the phase displacement point, thus reducing the expansion of the spectrum for the phase modulation signal.