A step down DC-DC converter is reduced in the cost by reducing a number of capacitors included. The DC-DC converter is provided with switches that connect (n−1) capacitors in parallel with each other and switches that connect the capacitors in series with each other. In phase 1, some switches are turned on to connect the capacitors in parallel between an output terminal and a ground voltage so that the capacitors are charged. In phase 2, other switches are turned on to connect the capacitors in series between the output terminal and an input voltage so that the capacitors are discharged. A stepped down output voltage, that is 1/n times of the input voltage, is obtained by alternating the phase 1 and the phase 2 until a stable state is reached.