To provide a switching power unit which is free of faults and is capable of being made compact and reduced in cost, by controlling a switching element without fail to be turned on and off, with respect to drop in the input voltage, in a switching power unit of digital control that uses a low-speed digital processor.

A digital pulse width modulation signal generator 44 provided in the digital processor 34 generates a first pulse width modulation signal V1, which is of specified cycles and in which specified maximum on-duty value is set. A comparator circuit 60 receives the input of a triangular wave signal V3, the inclination of which changes according to an input voltage Vin, and an output voltage control signal V4, and generates the second pulse width modulation signal V5. An AND circuit 62 outputs a second pulse width modulation signal V5, when the on-duty value of the second pulse width modulation signal V5 is below its maximum on-duty value and the first pulse width modulation signal V1 in case that the on-duty value of the second pulse width modulation signal V5 is at or over its maximum on-duty, as drive signals V6 to the switching element 18.