An in-vehicle power supply device includes an electric generator, a main power supply, a DC/DC converter, a power storage unit, and a controller for controlling the DC/DC converter. The DC/DC converter stores regenerative electric power generated by the electric generator, and discharges the stored regenerative electric power to the main power supply and the load. The controller stops the DC/DC converter when a charging-status value is not larger than a predetermined lower limit value. Additionally, the controller activates the DC/DC converter when the charging-status value is larger than the lower limit value and the vehicle ends deceleration; and activates the DC/DC converter when a speed of the vehicle is not lower than a predetermined speed and an accelerator position of the engine is not larger than a predetermined accelerator position even if the charging-status value is not larger than the predetermined lower limit value.