PURPOSE: To execute detection of electric energy, etc., by converting a load current to frequency, also counting the pulse and integrating the converted frequency, converting the integrated value to the integrated energy or its charge, and displaying it digitally.

CONSTITUTION: To the secondary side of a current transformer CT1 using a load circuit L1 as a primary coil, a rectifier RE1 is connected, and detected voltage is applied to a V-F converter VF1. From the converter VF1, a square wave of frequency being proportional to a load current, that is to say, a square wave of frequency being proportional to the used power of a load due to the fact that supply voltage of the circuit L1 is constant is generated. The output is frequency-divided DF1, is integrated CK1, is multipllied MC1 through a coefficient setting device KS1, and integrated energy of KW unit is outputted. Also, the used energy of KW/H unit is obtained through a frequency counter FC1, a multiplier MC2 and a coefficient setting device KS2, and is latched LT1, for instance, for 3 minutes. Also, a CPU1 operates the charge of the used power, and they are displayed digitally on an indicator DF1.