PURPOSE: To remove a noise component, by reversing the noise pulse signal based on a noise sensor and taking AND with the signal noise pulse based on a signal noise sensor.

CONSTITUTION: A noise pulse signal J having a prescribed length is generated from the detection noise of a noise sensor 20 through an amplifier 21 and a first pulse generator 22 and inverted by a NOT circuit 23 to form a noise inversion pulse signal L. The signal and noise detected by a signal noise sensor 24 generates a signal noise signal K through an amplifier 25 and a second pulse generator 26. The signals J, K are inputted to an AND circuit 27 and AND is taken to remove noise. An A/D converter 32 digitally converts the signal pulse signal M from the circuit 27 only when receives said signal M to display the number of signal pulses and a peak value.