PURPOSE: To discriminate insulation deterioration nondestructively by estimating the remaining insulation breakdown voltage of winding by using the sum of a value of the difference between starting mean discharge energy and current one and the difference between the starting logarithmic value of maximum discharge charge and the current one.

CONSTITUTION: A value W relating mean discharge energy and maximum discharge charge Qm are used to calculate a remaining insulation breakdown voltage VR by the equation, thereby discriminating insulation deterioration. In the equation, the increment Δtanδ of a dielectric tangent at a rated voltage and the increment ΔC/C0 of the quantity of static electricity are added together as a mean discharge- energyt relative value W to reduce constants K1 and K2 characteristic to insulation composition, thereby lessening errors.