PURPOSE: To reduce an orthogonal error or leakage of a carrier wave, by carrying out alternately the multiplication between the sine and cosine components of a base band signal and the sine and cosine components of the carrier wave through a digital arithmetic circuit in a period of an integer-fold value of the carrier wave.

CONSTITUTION: A base band signal (t) is fed to an input terminal 30 and then converted into a sine wave component sin(t) and a cosine wave component cos (t) through the waveform converting circuits 31 and 32 respectively. A frequency dividing circuit 42 produces the cosine component ±cosω0t and sine component ±sinω0t of a carrier wave to be fed to the phase selecting circuits 45 and 46 respectively, and either one of these two components is selected according to the polarity of the outputs of the circuits 31 and 32. Then a digital arithmetic circuit 50 executes the multiplication between the sin(t).cosω0t and cos(t).sinω0t and the time division multiplication in a digital way.