PURPOSE: To discriminate the occurrence of an internal fault by optically measuring the phenomenon that fine grains are produced owing to the thermal decomposition of an insulating material and the granularity or number distribution of floating fine grains in insulating oil varies.

CONSTITUTION: A definite quantity of insulating oil in a tank 1 is introduced as sample oil from piping 16 into a flow-rate control part 2, and then fed back from piping 18 to the tank 1 through piping 18 and a measurement part 15. The measurement part 15, having an incorporated optical measuring device for floating fine grains, to measure the size (r) and number N of the floating fine grains continuously to detect variations in granularity distribution. Measured values by the measurement part 15 are converted into electric signals, which after being amplified by an amplification part 20, are inputted to a comparison arithmetic part 21. The comparison arithmetic part 21 compares the input with the upper limit set value of graunlarity or number distribution set to a setting part 22 and, when a signal exceeding the set value is inputted, it sends an output signal to an output part 23 to discriminate the occurrence of an internal fault. This constitution enables to circulates the sample oil through a closed circuit, so the oil never comes into contact with the external air, so that the deterioration of insulation reliability due to sample collection never occur.