A power supply device includes a first switch section that can disconnect a 24-V power supply from a commercial alternating-current (AC) power supply and that operates with the voltage of 3.3 V, and a second switch section that can disconnect a voltage detection circuit from the commercial AC power supply and that operates with the voltage of 3.3 V. When a power saving mode is on, the 24-V power supply and the voltage detection circuit are disconnected from the commercial AC power supply by using the first and second switch sections, whereby the power supply device can be used in areas where different supply voltages are used, and at the same time reduction in power consumption in the power saving mode is achieved.