PURPOSE: To observe the abnormality of a waveform of light signals easily by converting the light signals inputted to an optical thyristor into electric signals and measuring the signals.

CONSTITUTION: During the operation (optical fibers 3-1∼3-n are connected as shown in broken lines) of the optical thyristor transducer, the ignition signals of a pulse generating circuit 1 are converted into light signals by means of light emitting elements 2-1∼2-n, and the light signals are supplied to the optical thyristors 8-1∼ 8-n through the optical fibers 3-1∼3-n. When inspecting gate light signals, the output terminals of the optical fibers are removed from the optical thyristors, and light signal detectors 11-1∼11-n consisting of light receiving elements 9-1∼9-n and amplifiers 10-1∼10-n are mounted at the output terminals of the optical fibers. The waveforms (the absolute value, pulse width, etc.) of the light signals can easily be measured because the output signals of the amplification circuits 10-1∼10-n correspond to the light signals at that time.