PURPOSE: To monitor the degree of deterioration of insulation, by measuring the component of the voltage which is proportional to the value of the leaked current of an AC rotary machine and accordingly obtaining the dielectric loss tangent of an insulator when the rotary machine is in operation working.

CONSTITUTION: The both-end voltage E2 of a neutral earthing resistance 4 of a three-phase AC generator is shifted by 90° by a phase shifter 7 and amplified at 8 up to -B-fold. The voltage added with the frequency voltage E1 obtained by applying 3-fold multiplication to the terminal voltage by an adder 9 is applied to a BPF10 to obtain a component E1A having the same phase as the voltage E2 in the voltage E1 to the output of the BPF10. At the same time, the output of the amplifier 8 is applied to a BPF11 to obtain a component E1B having a phase different from the voltage E2 by 90° at the output of the BPF11. Then the voltages E2, E1A and E1B of the output of a signal detecting circuit 13 are applied to an arithmetic processing part 14 to calculate tanδ0.