The present invention provides a DC/DC converter that can include a capacitor constituting a soft start circuit in a semiconductor integrated device. This DC/DC converter is comprised of a switching element for supplying power from a power supply (V_cc) to a load side output terminal (OUT) by switching, an error amplifier for amplifying the voltage from the load side output terminal (OUT), a differentiation circuit for differentiating the voltage from the load side output terminal (OUT) and adjusting the output current according to the value thereof, a soft start circuit comprising a constant current source and a capacitor, both of which are connected to the output terminal of the differentiation circuit, and a comparator for comparing the triangular wave voltage from a triangular wave generator (TRI) with the voltage from the soft start circuit or the output voltage of the error amplifier, and controlling the switching of the switching element according to the comparison output thereof.