A power supply apparatus having a soft start function that enables a startup without any overshoot within an appropriate startup time regardless of whether the load is heavy or light. Even when output direct current voltage Vo has not yet risen upon startup, switch circuit (4) is turned ON within a count time of timer (7), and clamping circuit (5) clamps error signal (Ve) of error amplifier (2), so that an excessive signal is prevented from entering PWM circuit (6), and electric power supplied to load (13) is controlled. Consequently, supply power as the power supply apparatus is limited, and any inrush current can be prevented from being generated. By controlling the count time of timer (7) when this error signal is clamped, it is possible to realize a startup within an appropriate startup time without any overshoot.