PURPOSE: To realize a frequency diversity with one receiver, by eliminating control for switching without providing any special switch circuit to a receiver of digital radio transmission system.

CONSTITUTION: The 1st FSK signal is delivered from an FSK signal generating circuit 3 of a transmitter 1 at a base station and centering on the frequency fT1 corresponding to a digital signal of the modulated input. On the other hand, the frequency fLT higher than the bit transmission speed of twice as much as the intermediate frequency fIF of a receiver 2 at a mobile station is delivered from a local oscillator 5 and mixed with the frequency fT1 through a mixer 6. Thus the 2nd FSK signal centering on the frequency fT2 is delivered from a BPF7, and the 1st and 2nd FSK signals are transmitted from a hybrid 4. These FSK signals are received at a mixer 9 of the receiver 2 and mixed with the oscillation frequency fLR of a local oscillation 8. Then frequencies fT1±fLR and fLR±fT2 are applied to a BPF10, and the 1st and 2nd FSK signals are demodulated at and after an intermediate frequency amplifier 11.