PURPOSE: To provide a voltage and current detecting device which can detect the input in a time of almost one cycle, even though the frequency is varied.

CONSTITUTION: This voltage and current detecting device is provided with a frequency measuring member 1 to measure the frequency of the input waveform, a timer 2 operating for a set time from the zero point of the input waveform, an A/D converter 3 to convert the analog amount of the input waveform into the digital amount, an operation member 4 to find the peak value of the input waveform by operating the formula Ex=Esin2πf't0 from the converting amount Ex of the A/D converter 3 at the time a set time t0 of the timer 2 passes and the measurement value f' of the frequency measuring member 1, and a deciding member 5 to detect the voltage and the current by comparing the peak value E of the operation member 4 with a standard value.