PURPOSE:To surely detect a covered snow state by providing a first photoelectric cell means for leading out a reference electric signal and a second photoelectric cell means placed in a steel toward and comparing electric signals from the first and the second photoelectric cell means. CONSTITUTION:A reference solar battery 20 (20a-20c) is provided on the top part of a solar battery panel 100, leads out an electric signal corresponding to its weather since the solar light input surface is no snowed by its structure, material and fitting direction, and applies said signal to an arithmetic part 31 through a signal line 23. Also, a solar battery 22 (22a-22g) leads out an electric signal of a smaller value than that of the electric signal from the battery 20 since the solar light incident surface becomes a covered snow state corresponding to a covered snow state in an insulator set and a steel toward arm member, and said electric signal is applied to the arithmetic part 31. Subsequently, in the arithmetic part 31, a ratio or difference of the electric signals from the batteries 20, 22 is calculated, a signal A corresponding to the covered snow state of the insulator set is led out, and in a comparator 32, the signal A and a reference threshold of a setting circuit 33 are compared, and a signal B is outputted.
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