Abstract: According to one embodiment, an insulation diagnostic system comprising: a peak-value acquisition circuit configured to acquire at least one peak value of a section corresponding to local discharge of a voltage signal acquired by at least one sensor that detects the voltage signal in a non-contact manner; a function acquisition circuit configured to acquire a calculation function for calculating charge amount related to the discharge based on at least two peak values acquired from the voltage signal that is detected by the sensor by applying a test voltage to a rotating electrical machine while the rotating electrical machine is stopped; and a charge-amount calculation circuit configured to calculate the charge amount related to partial discharge of the rotating electrical machine based on the calculation function and the peak value obtained from the voltage signal that is detected by the sensor during operation of the rotating electrical machine.