To provide an apparatus for detecting insulation deterioration of an a.c. electric motor which detects the insulation deterioration or the like abnormality of the driving a.c. electric motor by a corona discharge.

This detecting apparatus comprises an antenna 1 for detecting electromagnetic waves, and a corona discharge-detecting part 2. The corona discharge-detecting part 2 is constituted of a noise-removing part 4 extracting a high frequency component from a detection signal of the antenna 1 and removing a peripheral noise, strong, weak component-extracting parts 6, 7 carrying out an envelope detection through rectification of the signal after the noise is removed and extracting strong, weak components of electromagnetic waves of a corona discharge, a judging part judging that the electromagnetic waves result from the corona discharge from a ratio of a frequency component of a power source among the strong, weak components, and an alarm 10 generating an alarm on the basis of judgment.