A power supply device includes secondary battery cells disposed adjacent to each other and a separator that is interposed between the secondary battery cells adjacent to each other. The separator is made of a flexible material that has both a heat insulating property and restoring force such that the separator deforms when being pressed by each of the secondary battery cells and recovers an original shape of the separator. This configuration provides an improved heat insulating property between the secondary battery cells and prevents thermal propagation caused by thermal runaway. At the same time, this allows the separator to adapt to deformation of a secondary battery cell that has thermally swelled. In response to contraction of the swelled battery, the separator recovers its original shape to maintain a mechanical pressing force. This allows the power supply device to provide increased mechanical stability and maintain resistance to vibration and impact.