The DC/DC converter according to this preferred embodiment employs an nMOS transistor as a sWitch S1, and comprises a boost capacitor CB for starting up the nMOS
transistor. As described above, the boost capacitor CB is charged When the sWitches S1 and S2 are respectively OFF and ON, and is gradually discharged When the sWitch S1 is
ON. Accordingly, if the sWitch S1 remains in the ON state during a predetermined time period or longer, the voltage of the boost capacitor CB goes doWn. Consequently, the driver D1 cannot hold the sWitch S1 ON.