There is provided a motor insulation protecting device 200 comprising a motor 40 installed in a motor chamber 14; an inverter 16 that supplies an inverter voltage for driving the motor 40; and pressurizing means for monitoring an internal pressure in the motor chamber 14 and the inverter voltage of the inverter 16, and increasing the pressure in the motor chamber when an occurrence of discharge of the motor is predicted, wherein the pressurizing means comprises a pressure detection unit 50 that detects the internal pressure in the motor chamber 14; a voltage detection unit that is provided in a control unit 60, and detects the inverter voltage of the inverter 16; and a pressurizing mechanism 70 that increases the pressure in the motor chamber 14 in accordance with the output from the control unit 60 when the detected internal pressure is lower than a preset discharge occurrence prediction internal pressure, and the detected inverter voltage is higher than a preset discharge occurrence prediction inverter voltage.