PURPOSE: To improve reception sensitivity and to reduce distortion by using filters A and C whose gain characteristic are maximum at a frequency (a), 0 a frequency (c) and maximum at the frequency (c), 0 at the frequency (a), both being a frequency symmetrical to (b) and continuously change from frequency the (a) to the frequency (c).

CONSTITUTION: Notch filters 11 and 12 (upper limit carrier frequency f0 and lower limit one f2) having gain characteristics which are both symmetrical to f0, maximum at f1, transmission 0 point at f2 and maximum at f2, transmission 0 point at f1 are used. The filters 11 and 12 continuously changes in gain between f2 to f2, inputs the frequency-modulated signals of the occupied bandwidths f to f2 in which frequency modulations are performed for f0 and outputs each output 5A and 5B. The mark and space outputs 6A and 6B outputted by detecting these outputs 5A and 5B by detection circuits 2A and 2B are compared with an OR circuit 3, shaping circuits 3 and 4 compares them, they are faired and modulation signals 7 are obtained. Thus, reception sensitivity is improved and the distortion of modulation signals can be reduced.