A power supply device to be mounted on a vehicle includes a battery module, a power storage casing for accommodating the battery module, a DC/DC converter stepping down an output voltage of the battery module to a first voltage and a DC/DC converter stepping down the output voltage of the battery module into a second voltage. The battery module, DC/DC converter and DC/DC converter are fixed to the power storage casing. This structure provides the power supply device of small sizes capable of supplying electricity of three or more kinds of voltages.