PURPOSE: To detect synchronous information included in a QPSK signal surely even when a reproduced carrier is offset by a phase of a phase M signal by sweeping the phase in a specific range around the reproduced carrier phase while the synchronization of a reception system is not established.

CONSTITUTION: When the asynchronous state is decided by an asynchronous flag 24 of a synchronization detection section 14, a sweep signal generating circuit 25 generates a sweep signal 26, which is added to a phase error signal 16 of an tan-1 circuit 11. That is, an adder output 28 is +θ (θ is a bipolar value) and the reproduced carrier phase offset by the M phase PSK signal is swept for a specific range around the offset phase. As a result, in bringing the sweep range to, e.g., 45°/2, there is a point of time without fail where the swept and reproduced carrier phase is coincident with the true reference carrier phase even when the reproduced carrier is offset by 45°/2. The code error rate of the QPSK is improved at the coincident point of time and the synchronous information included in the QPSK signal is detected surely.