A data receiving system comprising an I base-band signal and a Q base-band signal which are used in a quadrature demodulation operation. Zero-crossing points of respective base-band signals are detected and a control signal is generated in response to each zero-crossing point. A phase-shift switching circuit alternately selects either the I base-band signal or the Q base-band signal in response to the control signal to generate an I/Q base-band signal. And, a demodulation operation is executed on the basis of the I/Q signal.