There are provided a switching power supply device performing a stable operation with fast response, a semiconductor integrated circuit device, and a power supply device.A capacitor is provided between the output side of an inductor and a ground potential. A first power MOSFET supplies an electric current from an input voltage to the input side of the inductor. A second power MOSFET turned on when the first power MOSFET is off allows the input side of the inductor to be of a predetermined potential. A first feedback signal corresponding to an output voltage obtained from the output side of the inductor and a second feedback signal corresponding to an electric current flowed to the first power MOSFET are used to form a PWM signal. The first power MOSFET has plural cells of a vertical type MOS-construction. The number of cells is 1/N. A detection MOSFET in which the gains and the drains are shared over the same semiconductor substrate with the first power MOSFET is provided to form a second feedback signal.