PURPOSE: To detect a signal with high accuracy and a simple configuration by discriminating the presence of detection of a signal based on both a change in a signal amplitude and lasting time of the amplitude.

CONSTITUTION: An input signal is inputted to an input buffer 11 and binarized and the result is inputted to a shift register 14 and shifted sequentially by a clock signal. The frequency of the clock signal and a the number of maximum stage of the shift register 14 are decided by time of noise superimposed on the signal. Through such a constitution, when an output of the registers 14 up to a prescribed number-th stage coincides at logical 1, a decoder 13 provides an output of signal detection state and it is latched by a signal latch 17. When an output of the registers 14 up to a prescribed number-th stage coincides at logical 0, a decoder 15 provides an output of signal non-detection state and it is latched by the signal latch 17. Thus, even when a noise component is close to or overlapped to a signal component in terms of a frequency band and an amplitude level, the signal is detected without malfunction by the output of the latch 17.