PURPOSE: To accurately detect a pattern of the generation of an internal corona even if there are external noises, by cumulating the detection of internal pulses synchronizing with a phase angle on a specified data area to external noises whose levels change.

CONSTITUTION: Pulses that external noises detected by means of a wave-form shaping circuit 8 and detecting pulses exist in mixed shapes are supplied to the input Y of a comparator 10, external noises from an antenna 11 are wave-form shaped 8' and fed to the input X of the comparator 10 and the output of the comparator is input to a cumulating processing portion 15. Potential dividing voltage from a high- tension bus 3 is detected 12, zero potential is further detected 13 while phase reference pulses are generated 14 from voltage wave-forms, and the potential and the pulses are each supplied to the processing portion 15. Internal corona pulses are cumulated by using each input datum in the processing portion 15, and the results are displayed 16.