Abstract: A non-destructive insulation test method and apparatus therefor, wherein in non-destructively testing the behavior of insulation by applying a high d.c. voltage to a test piece, stepped test voltages are applied to the test piece while observing partial discharges occurring within the test piece and monitoring a safe state. At the voltage application of each step, the applied voltage is controlled so that the current may maintain a fixed value until sufficient charge is fed to the test piece. The applied voltage is raised to a predetermined value in this manner so that the internal impedance of the test apparatus at charging can be regarded as substantially zero, whereby the rate of rise of the applied voltage to the predetermined value is hastened.