Provided is a DC-DC converter including a soft start circuit capable of prolonging a soft start time without increasing a capacitance used in the soft start circuit. A soft start is implemented by gradually increasing a limiting level of an inductor current or a reference voltage. The soft start time is adjusted by varying a frequency of CLOCK signals supplied to switch circuits. The soft start time may be prolonged without increasing a chip size because the capacitance does not need to be increased to prolong the soft start time.