To reduce loss in a step-down switching power source circuit which uses an N-type MOSFET as a switching element and has an overcurrent protecting circuit using a voltage drop due to on-resistance generated in the N-type MOSFET.

A switching element is constituted of an N-type MOSFET 11. A bootstrap circuit 15 generates a voltage which is to be applied to the gate of the N-type MOSFET. A control circuit 12 is provided with an overcurrent protecting circuit which detects a voltage drop due to an on- resistance of the MOSFET 11. When the detected voltage exceeds a first specified voltage, the protecting circuit gradually limits the on time of the MOSFET 11. When the detected voltage reaches a second voltage higher than the first voltage, the protective circuit makes the on time zero or sufficiently short. As a result, the loss of a stepdown switching power source circuit is reduced, and cost reduction and overheat protection of the switching element can be realized.