PURPOSE: To accurately and inexpensively monitor the voltage of a commercial power supply by achieving the same electrical resistance as that of a load circuit and at the same time connecting a voltage monitoring device to the secondary coil winding side of a power supply transformer and stopping operation when the load circuit is activated and performing operation when the operation stops.

CONSTITUTION: When no louver operation signal 801 for operating a louver motor 30 is inputted to a microcomputer 8, a relay operation instruction part 82 does not instruct the transmission of a conduction instruction signal 820 from an output port 821, a transistor 4 does not conduct and a connection point (m) becomes a high level for causing a transistor 6 to conduct and then a current with the same resistance as that of the relay 3 flows to a resistor 5. Also, when a signal 802 for operating the motor 30 is inputted to the computer 8, the transmission of the signal 820 from the port 821 is instructed, the transistor 4 conducts and the relay 3 is activated, the connection point (m) becomes a high level and the transistor 6 does not conduct, and no electric wave flows to the resistor 5, thus accurately and inexpensively monitoring the voltage of a power supply 10.