A step-up switching power supply unit for generating a predetermined output voltage through a feedback control of its output voltage based on a first detection voltage indicative of the output voltage. The power supply unit is adapted to monitor a second detection voltage associated with the output voltage that is compared with a reference voltage. Open-circuit malfunction arising from, for example, connection failures within the power supply unit can be detected by determining whether the second detection voltage is lower than the reference voltage or not. In the event that open-circuit malfunction has taken place, the switching signal controlling the power supply unit is promptly stopped, thereby preventing abnormal rise of the output voltage (i.e. over-voltage).