In one embodiment of the present invention, a voltage source is disclosed including a lower output impedance is connected to a capacitive load via a switch element and a voltage source including a higher output impedance is connected to the capacitive load via a switch element. Until a potential of an output terminal attains a reference potential, a comparator keeps the switch element in an ON state so that the voltage source writes a potential onto the capacitive load. When the potential of the output terminal exceeds the reference potential, the comparator causes the switch element to be in an ON state so that the voltage source writes a potential onto the capacitive load so as to have a predetermined potential.