PURPOSE:To inspect the deterioration of an insulator even when an electric wire is in a current supply state by converting the electric signal converted from an ultrasonic wave into a light signal and transmitting the light signal by the use of an optical fiber. CONSTITUTION:The probe 2 of an operation rod 1 is made to abut on an insulator S to be inspected in such a state that an electric wire is charged. At this time, an abutment member 12 abuts on the surface of the insulator S. The single pulse from a pulse oscillator 14 is incident on the region to be inspected in the insulator S as an ultrasonic wave by the probe 2 and this ultrasonic wave is reflected by the opposite surface of the insulator S to be received by the probe 2 and converted into an electric signal. The electric signal of the reflected wave converted by the probe 2 is converted into a light signal by an E/O converter 15 and this light signal is converted into an electric signal by an O/E converter 24 through an optical fiber signal wire 5 and the reflected wave is displayed on the display part of a flaw detector 3 through a coaxial cable 25. By this method, the advance state of the deterioration of the insulator can be accurately inspected in a current supply state.
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