The air conditioner control device/master unit receives alarms from the demand monitoring device as contact signals from the master unit via power line communication. The slave unit inputs this signal as a contact to the demand board. The demand board controls the output of the outdoor unit according to this contact, resulting in energy-saving operation. Because it is output control rather than ON/OFF, indoor air conditioners can be kept comfortable and energy-efficient. Two-way communication between the master unit and slave unit allows the slave unit’s alarm output signal to be input to the master unit.