To prevent the burning of an inverter even when a power supply is connected by mistake by installing a voltage detector on a circuit from the inverter to an electric shutter while inhibiting the starting of the inverter when the voltage detector detects voltage.

A voltage detector 13 is mounted between an inverter 12 and a changeover circuit 13, and the presence of voltage at the place of connection is detected and the signal is transmitted to a control section 11. An oscillation command circuit 11a controlling the oscillation of the inverter 12 and a starting inhibiting circuit 11b transmitting an inverter-starting inhibiting signal are mounted on the control section 11. Accordingly, when a test switch for an inverter performance test is depressed, the voltage detector 15 detects voltage by the reverse current of a temporary power supply 31 before an oscillation command is outputted from the oscillation command circuit 11a to the inverter by the control section, the inverter is left as it is stopped, and a current from the temporary power supply 32 is not reversed, thus preventing the burning of the inverter.