PURPOSE: To set a threshold level voltage to an optimum value even after synchronous supplement and to increase the probability of the synchronous detection of a desired wave, by controlling automatically the threshold level voltage to the optimum value in a synchronism searching process.

CONSTITUTION: A received signal Vi is applied to a correlator 1 and multiplied by a local reference signal from a code generator 6 and the product is supplied through an IF stage 2 to a demodulator 3 to be demodulated. The output of the demodulator 3 is applied to a synchronous detector 4; and a switch 12 is turned on when a code is searched for and a threshold level control circuit 11 is operated by the output signal of a comparator 7 to control a threshold level setter 9. The switch 12 is turned off by a signal generated when a synchronism deciding circuit 10 detects a desired received signal to control automatically the threshold level voltage to the optimum value while holding the level voltage even after synchronous supplement, improving the probability of the synchronous detection of the desired wave.