A power supply apparatus supplies a power supply voltage to a charge monitor that monitors charge states of rechargeable battery cells. The apparatus includes: a first capacitative element that supplies a power supply voltage to the charge monitor; a second capacitative element that is charged from the rechargeable battery cell and charges the first capacitative element; a switch group including a first switch that connects the first and second capacitative elements, and a second switch that connects the rechargeable battery cell and the second capacitative element; and a controller that controls the switch group. The controller repeats charging the second capacitative element by the rechargeable battery cell by connecting the rechargeable cell and the second capacitative element by the first switch, and charging the first capacitative element by the second capacitative element by connecting the first and second capacitative elements by the second switch.