A vehicle power supply device that is mounted in a vehicle that is provided with an internal combustion engine, the device comprising: a first power supply; a second power supply that is connected in parallel to the first power supply; a DC-DC converter that is connected between the first power supply and the second power supply; a determination device that determines whether or not a fuel supply to the internal combustion engine has been stopped; a deceleration detecting device that detects a deceleration of the vehicle; and a control device that controls the DC-DC converter, and a power generator that is linked to the internal combustion engine, wherein, when it is determined by the determination device that the fuel supply to the internal combustion engine has been stopped, the control device charges the second power supply using regenerative power that is supplied to the second power supply via the DC-DC converter, and, when a deceleration of the vehicle is detected by the deceleration detecting device, and it is also determined by the determination device that the fuel supply to the internal combustion engine has not been stopped, the control device is able to charge the second power supply using regenerative power that is supplied to the second power supply via the DC-DC converter, characterized in that the control device sets the output voltage from the second power supply to a first voltage when it is determined by the determination device that the fuel supply to the internal combustion engine has been stopped, and sets the output voltage from the second power supply to a second voltage which is smaller than the first voltage when it is determined by the determination device that the fuel supply to the internal combustion engine has not been stopped, and, when the control device changes the output voltage from the second power supply from the first voltage to the second voltage, the control device charges the first power supply by supplying power obtained from the difference between the first voltage and the second voltage to the first power supply. a first power supply; a second power supply that is connected in parallel to the first power supply; a DC-DC converter that is connected between the first power supply and the second power supply; a determination device that determines whether or not a fuel supply to the internal combustion engine has been stopped; a deceleration detecting device that detects a deceleration of the vehicle; and a control device that controls the DC-DC converter, and a power generator that is linked to the internal combustion engine, wherein, when it is determined by the determination device that the fuel supply to the internal combustion engine has been stopped, the control device charges the second power supply using regenerative power that is supplied to the second power supply via the DC-DC converter, and, when a deceleration of the vehicle is detected by the deceleration detecting device, and it is also determined by the determination device that the fuel supply to the internal combustion engine has not been stopped, the control device is able to charge the second power supply using regenerative power that is supplied to the second power supply via the DC-DC converter, characterized in that the control device sets the output voltage from the second power supply to a first voltage when it is determined by the determination device that the fuel supply to the internal combustion engine has been stopped, and sets the output voltage from the second power supply to a second voltage which is smaller than the first voltage when it is determined by the determination device that the fuel supply to the internal combustion engine has not been stopped, and, when the control device changes the output voltage from the second power supply from the first voltage to the second voltage, the control device charges the first power supply by supplying power obtained from the difference between the first voltage and the second voltage to the first power supply.