PURPOSE: To discriminate exactly a near-by frequency by a low sampling frequency and a short detection time by increasing or decreasing a value of a protective counter by a count value of the number of times of zero-crossing in accordance with a prescribed rule.

CONSTITUTION: An input signal is sampled by a constant period in a sampling circuit 1, and a polarity deciding result by a polarity deciding circuit 2 is stored in a storing circuit 3. A zero-crossing deciding circuit 4 compares the previous deciding result with the present deciding circuit, and when there is a variation in the polarity, an output is generated in a counting circuit 5, a prescribed sampling number is made one block and the number of times of zero-crossing is counted. Subsequently, a protective counter 6 increases or decreases a prescribed value in accordance with the number of times of detection of zero-crossing, a signal of a corresponding frequency is detected by a fact that a count value of the protective counter 6 has reached a prescribed value, and also, it is detected that the signal of the corresponding frequency has annihilated, by a fact that the count value has decreased to the prescribed value.