When a control circuit detects from a signal CPO4 that a battery voltage is less than a sixth reference voltage, a constant current operation in VFM control is performed with respect to a switching transistor and a synchronous rectification transistor in accordance with signals RVDET and CPO3. Furthermore, when the control circuit detects from the signal CPO4 that the battery voltage becomes equal to or greater than the sixth reference voltage, the constant current operation in PWM control is performed in accordance with a signal CPO2. When an output signal CVDET from a constant current/constant voltage switching detection circuit becomes high level, operation control with respect to the switching transistor and the synchronous rectification transistor is switched from the PWM control of constant current operations to the PWM control of constant voltage operations in accordance with a signal CPO1.