To provide a power factor correction type switching power supply unit which uses a transient response improvement circuit of a simple structure, prevents side effects caused by the transient response improvement circuit, and stabilizes an output voltage.

A soft-stop overvoltage protection circuit 90, into which a soft-stop overvoltage detection voltage Vfb proportional to a direct current output voltage is input, reduces the output of a multiplier 24 in accordance with the soft-stop overvoltage detection voltage Vfb when the soft-stop overvoltage detection voltage Vfb exceeds a first threshold value. An overvoltage protection circuit 100, a second threshold value higher than the first threshold value being set, compulsorily turns off a switching element 6 by outputting an overvoltage detection signal when the soft-stop overvoltage detection voltage Vfb exceeds the second threshold value. The soft-stop overvoltage protection circuit 90 compulsorily increases the output of a voltage error amplifier circuit 14 when the output voltage decreases. When the output of the voltage error amplifier circuit 14 increases suddenly and the output voltage rises excessively, the soft-stop overvoltage protection circuit 90 decreases the output of the multiplier 24, thus curbing the rise of the output voltage.