PURPOSE: To enable detection of abnormal waveform in the vicinity of the limit of a dynamic range via a sine waveform and also detection of waveform except for the sine waveform by counting differentiated waveform of a signal to be measured for predetermined time.

CONSTITUTION: Signals a, a' to be measured are diferentiated by a differentiator 1 to become differentiated signals b, b', respectively, which are shaped via a waveform shaper 2 to thereby become digital waveform signals c, c', respectively, which are then applied to a discriminator 3 to be thus discriminated. In this discriminator 3, the signals c, c' are again differentiated to become differentiated signals, which are full-rectified to thereby form signals d, d', which are counted for predetermined time to thereby discriminate the difference between the signals d and d' to judge the signal d as good and the signal d' as bad. It is noted that the signals may be applied to a full-wave rectifier to thereby discriminate them whether good or not.