To provide a ground monitor of electrostatic ground system.

A detection circuit is provided, which is provided with a polarity test circuit 10, a step-down rectifying circuit 20, a ground test circuit 30 and an alarm device 40. Commercial power is drawn by the polarity test circuit 10, the polarity of power source circuit is tested. Standard zero potential circuit of ground line or neutral line in power source circuit is separated to be a reference potential of the ground detection. Also by converting commercial power drawn from the polarity test circuit 10 by the step-down rectifying circuit 20 to direct current power source with low voltage to provide for operating the ground test circuit 10. By comparing with the test circuit potential of aground bus to detect whether or not the ground bus, whether or not the ground bus really grounds. If a ground bus is not installed, an alarm driving signal is sent to the alarm device 40 and thus ground state of the ground bus is exactly detected.