PURPOSE: To prevent malfunction due to the effect of a surrounding noise and other signals.

CONSTITUTION: An amplifier 1, a broad band filter 2, an amplifier 3 and an AGC circuit 4 are used to make a noise level constant. A limiter 5 is used to limit a maximum level of a signal and a narrow band filter group 6 passes only a signal of a selected frequency. A comparator group 8 provides a detection signal respectively when an output from an integration circuit 7 is a setting level or over respectively. A signal generator group 9 provides an output of a signal with a frequency corresponding one to one to each specific frequency. A gate circuit group 10 uses an output from the corresponding comparator 8 as a gate signal to control an output from the signal generator 9. Thus, even when a level of an input signal is high, the S/N of the signal is kept constant and malfunction due to a noise or a signal of other frequency is prevented.