upon increase of the modulated input 101 the signal vectors progressively increase away from the circles as indicated by arrows away from the origin of the coordinate system so that the outputs of the ex ors 7 and 8 provided by logically processing the a d converted outputs x 1 and x 2 and y 1 and y 2 respectively become logical 34 0 34 in all the conditions this reduces the gain of the if amplifier 1 to control the leading ends of the signal vectors of the modulated input 101 to return to the original circled positions conversely upon decrease of the modulated input 101 the leading ends of the signal vectors move toward the origin that is in the opposite direction to the arrowed direction whereby all the outputs of the ex ors 7 and 8 become logical 34 1 34 to increase the gain of the if amplifier 1 in this manner the leading ends of the signal vectors of the modulated input 101 are always controlled to the circled positions thereby maintaining the optimum operation condition in the above description the demodulation sensitivity or the gains of the bb amplifiers have been assumed to remain unchanged for both the inphase and quadrature phase components when the gain of the bb amplifier 4 increases beyond the gain of the bb amplifier 3 the situation may be considered equivalent to one wherein the leading ends of the signal vectors have moved to dots in.