PURPOSE: To obtain a signal identifier capable of identifying sufficiently a signal from other input signals with simple constitution even if an input signal within a limited frequency is used by using the storage function of a flip-flop.

CONSTITUTION: When a device is desired to be started, the 1st input signal is fed to a phase comparator 1 from a signal generator (not shown) and compared with a frequency of the 1st oscillating signal from a variable oscillator 2. When both the frequencies are coincident, the level of a terminal 12 goes to a low level and the flip-flop 5 is brought into standby state. Since a transistor 7 is turned off at the same time, the oscillating frequency of the variable oscillator 2 is decided by a capacitor 8 and a variable resistor 10, and the 2nd oscillation signal is fed to the said phase comparator 1. On the other hand, the 2nd input signal a different in frequency from that of the 1st input signal is fed to the comparator 1 afterward. When the both are coincident, a coincident signal is outputted, the flip-flop 5 in standby state is brought into the set state and an output to control the device (not shown) is transmitted.