A power source device of present disclosure includes: a first power storage element; a second power storage element that is connected in parallel to the first power storage element and that has a lower internal resistance than an internal resistance of the first power storage element and a lower storage capacity than a storage capacity of the first power storage element; an opening and closing part that is connected between the first power storage element and the second power storage element and that switches between a disconnection state and a connection state; a charge circuit that is connected to an input route of the first power storage element and that performs a step-down operation; a discharge circuit that is connected to an output route of the second power storage element and that performs a step-up operation; and a controller that controls operations of the opening and closing part, the charge circuit, and the discharge circuit.