PURPOSE: To prevent the deviation of a tuning frequency based on a change in the ambient temperature by providing the reception circuit side with a couple of switched capacitor filters(SCF) separating a transmission signal into two reception signals different in frequency.

CONSTITUTION: A couple of SCFs 22, 23 are provided to a reception circuit 5 to separate the frequency of a received FSK (frequency transit modulation) signal 21S and the sampling clocks 42S, 43S of the SCFs 22, 23 are obtained by utilizing a crystal oscillator 11 and programmable counters 12, 13. Thus, the SCFs 22, 23 acting like a resistor equivalently do not include a resistance or inductance element having a large temperature dependency and the sampling clocks 42S, 43S are kept easily and accurately to a degree from a multiple of 50-100 of the tuning frequency of the SCF. Thus, the data transmission circuit of the FSK system with high frequency selectivity whose frequency selectivity change is less than 1% over a wide temperature range is obtained without complicating the circuit constitution.