PURPOSE: To allow visual intuitive recognition of the cause of trouble by displaying a current data without affixing any polarity sign when the phase angle of current with respect to the higher harmonic voltage of same order is within a predetermined range otherwise displaying the current data affixed with a negative polarity sign.

CONSTITUTION: When the phase angle of current with respect to voltage is within ±90°, for example, the current flows from the supplier side (power supply) to consumer side (load). When the phase angle deviates from the range of ±90°, the current flows reversely, i.e., from the consumer side to the supplier side. Consequently, when the power of a distribution system is measured upon occurrence of trouble and a negative polarity sign is affixed to a higher harmonic data of some order, that distribution system is connected with some load generating and externally feeding an electromotive force of noise and a decision can be made that the cause of trouble is present on the factory side. When the negative polarity sign is not affixed, a decision can be made that the cause of trouble is present on the supplier side.