PURPOSE: To derive a peak value and a crest factor in a real time by holding the peak value and the peak factor of an AC current flowing to a non-linear load, while updating them at every half cycle, and displaying the highest one in holding values in a prescribed period.

CONSTITUTION: A current flowing through a computer 4 and its voltage are detected by a current transformer 11 and a transformer 12, respectively, and the output of the current transformer 11 is provided to a peak value holding circuit 14 and an effective value arithmetic circuit 15 together with the output of the transformer 12 through a current/voltage converter 13. Subsequently, a current effective value from the circuit 15 and a current peak value from the circuit 14 are inputted to a holding circuit 16, and the circuit 16 calculates a peak factor at every half cycle and holds it, while updating its result. Also, one of outputs of the circuit 15 or 16 is selected by a changeover switch 17, sampling is executed in a period prescribed by a sampling time setting circuit 18, and after the sampling time is ended, the maximum value in its period is displayed on an indicator 21.