PURPOSE: To demodulate data without catching another carrier wave even in the case of narrow carrier wave spacing and without causing erroneous synchronization even in the case of wide band transmission by measuring time after the switch-on of power supply, and securing a normal frequency sweep section by sweeping local frequency when a certain time elapses after that.

CONSTITUTION: A timer 2 to measure time elapsed from the switch-on of the power supply 1 commencing immediately after the switch-on of the power supply 1, and a switch 4 to supply or stop the output of a sweeping oscillator 3 to a demodulator 5 are provided. Then, after the lapse of the stabilization time of the sweeping oscillator 3 set beforehand, the timer 2 outputs a control signal so as to turn on the switch 4.