PURPOSE: To make the entire device small, and the weight light and to reduce the cost by calculating signals directly according to the expression equation representing the phase modulation of a carrier by a modulation wave in the modulation theory so as to execute the phase modulation.

CONSTITUTION: A phase modulation circuit is provided with an adder 1, a multiplexer 2, a sinusoidal function reader 3, an A/D converter 4, a D/A converter 5, a ring counter 6, input terminals 11-1N, 41, 61, and output terminals 21-2N, 20, 30, 40, 50. Through the circuit constitution above, the circuit components except the A/D converter 4 and the transfer 5 are integrated as an LSI. Then the carrier is phase-modulated by using a trigonometric function and other relevant values so as to calculate electronically directly a carrier signal and a modulation signal time by time. Thus the phase modulator is made small in size, the weight light and the cost is reduced.