PURPOSE: To provide a modulation/demodulation system and a modem which can stably generate a reference signal for synchronizing detection and also can demodulate the data in simple circuit constitution.

CONSTITUTION: A distributor 607, a synthesizer 608 and a frequency converter 610 generate a reference signal of the frequency equal to the carrier frequency fc from an input reception pilot signal. A phase shifter 609 shifts the phase of the output signal of the synthesizer 608 by an extent equal to the amount that is controlled based on the output signal of a phase shift amount setting circuit 614 and supplies the shifted signal to the converter 610. A synchronizing detection circuit 604 applies the synchronizing detection to an IF reception signal based on a synchronizing detection reference signal. Then the circuit 614 sets the phase shift amount of the phase shifter 609 within a fixed time when the data value is kept at a fixed level since a system was started based on the output base band signal of an LPF 611.