A vehicular power supply device includes an inverter, a converter, a driving motor, an electric device group, a first electric storage unit to be coupled to the driving motor via the inverter, a second electric storage unit to be coupled to an electric device group via the converter, an electric power converter, first and second switches, and a switch controller. The electric power converter is provided between the two electric storage units and couples them in parallel with each other. The first switch is provided between the first electric storage unit and the electric power converter and is controllable between on and off states. The second switch is provided between the second electric storage unit and the electric power converter and is controllable between on and off states. The switch controller controls the first and second switches based on a state of charge of the first electric storage unit.