PURPOSE: To prevent an erroneous detection by controlling the partial discharge test such that partial discharge detection output generated in a partial discharge detector takes a minimum value when a reference signal is applied to the cable to be tested and performing the partial discharge test of the cable under this condition.

CONSTITUTION: When a partial discharge test is performed, the reference signal output of a reference pulse generating circuit 12 is fed into the core wire 2 of a cable 1 to be tested via a capacitor 13. When, based on the reference signal, signals are generat ed in first and second measuring electrodes 5 and 6, respectively, in proper electrode conditions, the output of an output coil 7b is zero. When the electrode conditions change due to change in the diameter of the cable 1 or the like, a reference output S0 corresponding to the polarity of a resultant differential signal and a level difference is produced in a partial discharge detector 10. Then, an electrode condition controller 11 is operated on the basis of the output S0 and the electrode conditions of the electrodes 5 and 6 are controlled by control output signal S1 and S2, respectively. Therefore, even when the diameter of the cable 1 changes, the equilibrium conditions of the electrodes 5 and 6 are controlled in response to the change in the diameter, so that an erroneous detection is prevented.