PURPOSE: To attain to enhance a S/N, by making it possible to detect high frequency, which is generated by insulation inferiority, at every divided frequency band.

CONSTITUTION: The radiation magnetic field emitted from power machinery is detected by a loop antenna 1 and amplified to a definite level by a preamplifier 2 to be inputted to a main body3 to reduce the effect of noise. The signal further amplified by a high frequency amplifier 4 is amplified by the band amplifier selected by cooperated change-over switches 5A, 5B. For example, when a band amplifier 6 is selected, only a signal of 70W110mHz is amplified and subjected to frequency conversion to be inputted to a first intermediate frequency amplifier 21. Hereinafter, intermediate amplification is performed while the amplified signal is detected by a detection circuit 24 and amplified by a DC amplifier 27 to be displayed on an indication meter 28. Because there is definite relation between the excellent frequency of power machinery and the adapted voltage thereof, by selecting band amplifiers 6W8, measurement can be performed in an enhanced S/N.