A power supply includes a transformer, a switching unit configured to drive the transformer at a primary side of the transformer, a voltage adjustment unit configured to adjust voltage of a capacitive element at a secondary side of the transformer, at the primary side thereof, and a plurality of constant voltage members connected across the terminals of the capacitive element at the secondary side of the transformer, wherein the power supply generates voltage having a predetermined polarity during the period before the voltage of the capacitive element at the secondary side of the transformer becomes a predetermined voltage by the voltage adjustment unit, and generates voltage having a polarity opposite to the predetermined polarity after the voltage of the capacitive element at the secondary side becomes the predetermined voltage.