In order to achieve the spectrum economy and reduce the power consumption of a transmitter and reduce the error rate at a receiver and simplify receiver circuitry, at least one bit stream to be transmitted is quadrature modulated in a manner that the signal points do not define a trace on a complex amplitude plane which passes through the origin of the plane coordinates or in the vicinity thereof. The received signal is hard limited and frequency-discriminated to determine the instantaneous angular velocity of the received signal and is integrated to detect the phase variation between two consecutive received signals, and thereafter is decided with respect to the phase variation according to the decision threshold levels 0 and .+-..pi. radians.