PURPOSE: To omit an exclusive frequency selection filter by variably controlling the selected central frequency of a filter which is varied at the seleced central frequency in response to an input signal.

CONSTITUTION: Modulators 31W3n respectively AM-modulate the modulated waves fm1Wfmn of oscillators 11W1n with the carrier wave fc of an oscillator 2, and a selector 4 selects ATC signals f1Wfn applied from the modulators 31W3n under train position conditions and route opening conditions (TR conditions). On the other hand, a selector 11 selects a multiplying factor control signal fv corresponding to the ATC signal fm selected by the selector 4 by the TR condition input and outputs it to a receiver 7. The receiver 7 is set at the selected central frequency of a switched capacitor filter 78 to the modulated wave signal fm by a control signal fv, extracts the signal fm and supplies it to a track relay TR.