PURPOSE: To shorten detection time and to sufficiently eliminate a high AC induced voltage by providing an operational amplifier to reduce an in-phase voltage included in a ground detection input signal at the entry side of a low- pass filter.

CONSTITUTION: When the AC induced voltage is applied on a ground detection circuit, almost the same voltages are applied on input terminals 2 and 3. The voltages are voltage-divided with detection resistors 5 and 6, and are applied on the plus input side of the operational amplifier 32. Meanwhile, the same voltage is voltage divided with resistors 30 and 31, and are applied on the minus side of the operational amplifier 32. At this time, since the detection resistors 30 and 31 are provided high resistance, only an AC signal can be inputted to the operational amplifier 32 without affecting on the current change of the detection resistors 5 and 6 at an detecting side. Difference between the input at the plus side and that at the minus side can be outputted from the operational amplifier 32.