A switching transistor has a source connected to the ground and a drain connected to the first terminal. The other end of an inductor whose one end is to be connected to the first terminal is connected to the second terminal. An input voltage is applied to the third terminal. An auxiliary transistor is disposed between the second terminal and the third terminal. A first soft start circuit generates a first soft start voltage that increases with lapse of time, and a second soft start circuit generates a second soft start voltage that increases with a delay to the first soft start voltage. An error amplifier controls the gate voltage of the auxiliary transistor on the basis of the error voltage between the output voltage and the first soft start voltage.