To provide a power source device capable of immediately shutting off power supplied to the load, when a rectangular wave is supplied to the power source device from an AC power source, and to provide an image forming apparatus that uses this power source device.

The power source device includes a serial circuit, having a relay contact with a triac 109 connected to an AC power source 114 via a required load; square-wave detecting means 105 and 107 that detect the supply of square wave by the AC power source; and a power shut-off means 107 for stopping the drive of the triac and relay, when the supply of a square wave is detected by the square-wave detecting means, thereby shutting off power supplied for the required load.