POWER SUPPLY DEVICE

The power supply device includes a plurality of battery cells each having a square outer shape, a plurality of separators for insulating the adjacent battery cells, and a restraining member that assembles the plurality of battery cells and the plurality of separators. Each of separators has heat insulating properties. Further, each of separators is disposed between the plurality of battery cells. Further, each of separators includes contact portion that comes into contact with adjacent battery cell, and thin portion formed thinner than contact portion.