PURPOSE: To realize phase synchronization detection without use of an external connector by generating a sweep signal of a voltage controlled oscillator based on the phase comparison between a carrier obtained in a PLL loop having the voltage controlled oscillator and an N-phase modulation signal and on the result of amplitude comparison between the comparison output and a reference voltage.

CONSTITUTION: An N-phase modulation signal (a) is divided into two by a distributer 5, part of them is inputted to a 1st N multiplier 6, where the signal is subject to N multiple and a carrier signal (g) not subject to phase modulation is obtained. On the other hand, when a frequency of the N-phase modulation signal (a) exists in the lock range of a PLL loop comprising a phase detector 1, a phase error detector 2, a 1st low pass filter 3 and a voltage controlled oscillator 4, an output signal of the voltage controlled oscillator 4 starts locking of the frequency and phase rapidly. An output signal (h) of the phase comparator 8 passes through a low pass filter 9 and identified with a reference voltage signal (j) from a reference signal generator 10 at an amplitude processing 11. The reference voltage signal (j) is set properly to obtain a detection signal (k) discriminating the synchronizing state and the asynchronizing state is obtained.