PURPOSE: To make it possible to calculate an impedance square value having no frequency error, by performing predetermined operation by using current and voltage data which are sampled at a predetermined cycle and mutually set at 90° intervals.

CONSTITUTION: When Ai→Ai-1, Ai-1→Ai-2 and =θk are set in the frequency characteristic of a reactive component power formula Ai.Bi-1-Bi.Ai-1=Ap.Bp. Sin.sin(θk), Ai-1.Bi-1-Bi.Ai-2=Ap.Bpsin(θk).sin(θk) is formed. Herein, when Ai=Bi is set to fix phase relation, x=Ai-12-Ai.Ai-2=A2psin2(θk), y=Bi-22-Bi. Bi-2=B2Psin2(θk) are obtained. From this, y/x=(Bi-12-Bi. Bi-2)/(Ai-12-Ai. Ai-2)=B2p.sin2(θk)/A2psin2(θk)=z2 are formed and the judgement of a level or the measurement of a ratio of x and y can be performed by y<x.k2 or y>x. K2 (wherein K is an integer constant).