Ground fault sensing device

A ground fault sensing device for a drive system that converts direct current electrical power from a direct current power supply (30) into alternating current electrical power and drives an electric motor (10) with that alternating current electrical power, comprises: an extraction circuit (41) that is connected to a power supply line of the direct current power supply (30) via a coupling capacitor, that detects an amplitude wave that is generated when an alternating current side including the electric motor (10) is suffering a ground fault, and that extracts the envelope of the amplitude wave; a measurement circuit (42) that measures the voltage level of the envelope; and a determination circuit that makes a decision as to whether or not a ground fault has occurred on the alternating current side by comparing together the voltage level measured by the measurement circuit (42) and a ground fault decision value.