A switching power supply circuit attaining high power conversion efficiency and a low operation start lower-limit voltage is provided. The switching power supply circuit in accordance with the present invention comprises an input terminal to which an input voltage is input; an output terminal from which an output voltage is output; a choke coil and a rectifying device, connected in series between the input terminal and the output terminal, to which the input voltage is input and from which the output voltage is output; a voltage step-up switching device, one terminal of which is connected between the choke coil and the rectifying device and another terminal of which is connected to the grounding point of the circuit; a power supply control circuit, to which the output voltage is input as a drive power supply, for controlling the voltage step-up switching device and the rectifying device; a transistor connected between the input terminal and the output terminal in parallel with the series connection of the choke coil and the rectifying device; and a differential amplifier, to which a first voltage derived from the input voltage and a second voltage derived from the output voltage are input, for turning ON the transistor when the first voltage is higher than the second voltage and for turning OFF the transistor when the first voltage is lower than the second voltage.