PURPOSE: To provide the orthogonal degree stablizing circuit for MODEM in multi-channel system which can exactly multiply a base band signal and a carrier frequency signal making the phase different at 90° in the case of performing orthogonal amplitude modulation due to a direct modulation system.

CONSTITUTION: A signal at a carrier frequency (f) is outputted from a synthesizer 11 by a frequency setting signal and when the phases of output signals S11 and S12 from a 90° hybrid circuit 2 are not made different at 90° by this output signal, data are read out of a ROM by the frequency setting signal for canceling the phase difference. These data are supplied through a D/A converter to a variable capacitor so as to perform control for changing the capacity of the variable capacitor into capacity to obtain the phase difference at 90°, the Ich base band signal and the signal S11 are multiplied by a mixer 3, and the Qch base band signal and the signal S12 can be multiplied by another mixer 4.