e g 1 a when the detected current is greater than the predetermined value u03b1 e g 1 a the overshoot preventive circuit 63 continuously raises the gate voltage of the p mos transistor 41 in response to the detected current so that the current supplied from the p mos transistor 41 is successively reduced to the level smaller than the predetermined value u03b1 e g 1 a when the above described operations are performed the voltage vo is changed in a way as shown in fig1 as a result the voltage vo can be prevented from the undershooting during the time the mode is changed from the sleep mode to the normal operation mode and from the overshooting during the time the mode is changed from the normal operation mode to the sleep mode in addition the overshoot preventive circuit 63 also prevents an excessive current flowing from the p mos transistor 41 in the sleep mode when a short circuit occurs in a load connected to the smoothing circuit 32 with this the power supply apparatus 1 can prevent an excessive current output from the dc to dc converter 2 in the sleep mode as described above in.