An N-channel MOS transistor is used to configure a DC--DC converter having good characteristics without providing a particular voltage circuit which outputs a voltage higher than an input voltage. An N-channel MOS transistor 11 and a choke coil 12 are connected in series between a negative side of a direct-current power supply and a negative side of the output capacitor 8. A PWM circuit 14 applies pulse signals to a gate of the MOS transistor 11 based on the input voltage divided by the capacitors 9 and 10. The pulse signal of the PWM circuit 14 is formed by the voltage corresponding to the output voltage of the output capacitor 8, and the pulse signal is inputted to the gate of the N-channel MOS transistor 11 through a photocoupler 16.