To uniformly cool a battery module as a whole by making temperature differences of the battery modules less and making temperature differences in a long and slender battery module itself less.

The power supply device houses a plurality of battery modules 2 in a parallel state in a holder case 1 and has it forcibly blown with a blower mechanism. In the holder case 1, an inner shape of a housing room 9 housing each battery module 2 is made larger than an outer dimension of the battery module 2 to arrange a cooling gap 10 between them. The power supply device sends air forcibly to the cooling gaps 10 in an axis direction of the battery modules 2 with the blower mechanism 3 to cool the battery modules 2. The inner shape of the housing room 9 is made in a shape approaching the surface of the battery module 2 in a blowing direction of the air, so that flowing speed of the air blown on the surface of the battery module 2 is made higher toward the blowing direction of the battery module 2 to curtail temperature differences of the battery module 2 in the axis direction.