A low-voltage operating block and a high-voltage operating block are controlled at the same soft start voltage as that employed at the time of start-up. At a point in time at which an output DC voltage has reached an expected value, a control signal of the low-voltage operating block is switched to a control signal of the high-voltage operating block. As a result, switching is smoothly performed without involvement of occurrence of a variation in the output voltage before and after switching operation. In a switching power supply unit having a low-voltage operating block and a high-voltage operating block, which are intended for producing a higher DC voltage from a low power supply voltage, it prevents occurrence of a rush current and a start-up failure, which would otherwise be caused by switching operation.