There is provided a switching power supply circuit which receives a voltage from a direct current voltage source and supplies a direct current power to a load part. The circuit includes a switching element, a current detection circuit which is serially connected to the switching element, and which converts a current flowing in the switching element into a voltage, a control circuit which outputs a switch driving signal to control an ON/OFF operation of the switching element such that a current flowing in the load part becomes constant, a reference voltage generation circuit which generates a reference voltage proportional to an ON duty of the switch driving signal, and an error amplification circuit which outputs error information between a voltage output from the current detection circuit and the reference voltage. The control circuit adjusts the ON duty of the switch driving signal based on the error information.