A first constant-current source supplies a charging current to a second terminal of a first capacitor in which a first terminal is grounded. A first terminal of a second capacitor is grounded, and a first switch is provided between the second terminal of the first capacitor and a second terminal of the second capacitor. A second constant-current source supplies current to the second capacitor, and a second switch is provided on a current path of the second constant-current source. A third constant-current source draws out current from the first capacitor and the second capacitor. A switch controller turns on the first switch and turns off the second switch when an output voltage appearing in the second terminal of the first capacitor reaches a predetermined upper-limit voltage. The switch controller turns off the first switch and turns on the second switch when the output voltage drops to a predetermined lower-limit voltage.