An insulation resistance monitoring device for monitoring an insulation resistance of an object to be measured, a normal value of the insulation resistance being about 100 MΩ, the object being connected to a ground through a ground line and driven by a voltage of about 200 to 480 VAC applied through a power supply line, wherein the insulation resistance monitoring device comprises: a current detecting unit that is configured to be connected between the power supply line and the ground line of the object to be measured and configured to detect a current flowing through the insulation resistance of the object to be measured; a voltage generating unit that is configured to be connected between the power supply line and the ground line of the object to be measured and configured to generate a voltage of about 500VDC to supply the voltage to the object through the power supply line; and a switch configured to open and close a connection path between the power supply line or the ground line of the object to be measured and the voltage generating unit, the device being further configured to have the insulation resistance of the object measurable by a mega tester connected between the power supply line and the ground line, wherein, when the connection path is closed by the switch in a state where a power supply through the power supply line to the object is stopped, and the voltage generating unit generates a voltage of about 500VDC to supply the voltage to the object through the power supply line, the current detecting unit is configured to perform an output indicating an abnormality if the insulation resistance value calculated based on a current detected by the current detecting unit is outside a range of the normal value, wherein, when the insulation resistance of the object is measured by a mega tester connected between the power supply line and the ground line for legal inspection, the connection path is opened by the switch in a state where a power supply through the power supply line to the object is stopped, wherein the switch has an off resistance of 100 MΩ or more when the connection path is opened. a current detecting unit that is configured to be connected between the power supply line and the ground line of the object to be measured and configured to detect a current flowing through the insulation resistance of the object to be measured; a voltage generating unit that is configured to be connected between the power supply line and the ground line of the object to be measured and configured to generate a voltage of about 500VDC to supply the voltage to the object through the power supply line; and a switch configured to open and close a connection path between the power supply line or the ground line of the object to be measured and the voltage generating unit, the device being further configured to have the insulation resistance of the object measurable by a mega tester connected between the power supply line and the ground line, wherein, when the connection path is closed by the switch in a state where a power supply through the power supply line to the object is stopped, and the voltage generating unit generates a voltage of about 500VDC to supply the voltage to the object through the power supply line, the current detecting unit is configured to perform an output indicating an abnormality if the insulation resistance value calculated based on a current detected by the current detecting unit is outside a range of the normal value, wherein, when the insulation resistance of the object is measured by a mega tester connected between the power supply line and the ground line for legal inspection, the connection path is opened by the switch in a state where a power supply through the power supply line to the object is stopped, wherein the switch has an off resistance of 100 MΩ or more when the connection path is opened.