This invention realizes control without using a current-detecting element to prevent over current from flowing into a switching element of a switching converter in response to an input primary voltage from a small-capacity DC power source. The switching converter includes an FET 30 that turns the input primary voltage from the DC power source 1 on and off, and a choke coil 31 that is charged or discharged according to turning on/off of this FET 30. A driver 13 drives the FET so that an output voltage detected by an output voltage detector 9 is converged to a target voltage. A means for calculating a current I that flows into the FET 30 based on a duty ratio of the input primary voltage and the FET 30 and the inductance of the choke coil 31. The driver 13 drives the FET 30 at a duty ratio set so as to prevent the estimated current value I from exceeding a permissible current value or stops driving when the estimated current value I is equal to or more than the permissible current value.