A switching power supply unit is provided, in which a DC input voltage can be detected even if switching operation of the power supply unit is stopped. A switching power supply unit includes: a power supply main section switching a DC input voltage inputted from a first power supply to convert the DC input voltage into an AC voltage, and outputting a DC output voltage into a second power supply, the DC output voltage being obtained by transforming and rectifying the AC voltage; and a voltage detection section having a voltage detection transformer, one or more switching elements, and detection signal lines. The voltage detection transformer includes a first transformer coil as a primary winding being intermittently applied with the DC input voltage in response to on/off of the switching element, and a second transformer coil as a secondary winding being connected to the detection signal lines.