To provide a counter for an electric signal resistant to noise and free from error count.

In the counter which is provided with a diode 1 for detecting an input signal S and a count switch 3 operating with the output of the diode 1 as a control signal and performs counting based on switch current flowing by the switch 3, a field effect transistor having pinch off voltage through which current flows in response to output by operation with voltage equal to or lower than the threshold of the diode 1 is used as a switch 3. In the characteristic of a general diode, a threshold voltage is defined as 0.6 V, and the diode is off when the voltage is lower than the threshold voltage. That is, it is a matter of common knowledge to regard the state as off state since voltage equal to or lower than the threshold voltage belongs to a dead zone. However, even in the case of being in this fine voltage zone of voltage equal to or lower than the threshold, the directionalities of forward current (forward I/V characteristic) and reverse current (reverse direction I/V character) appear clearly in a micro view. By making this zone usable for counting operation, the threshold is made to be fine voltage so as to realize normal counting without regard to noise.