A power supply apparatus includes a battery cell, a voltage conversion circuit connected to the battery cell, a first housing that fixes the battery cell in a manner to enable heat transfer, a second housing that fixes the voltage conversion circuit in a manner to enable heat transfer, and a low thermal conductive layer interposed between mating surfaces of the first housing and the second housing having a thermal conductivity smaller than a thermal conductivity of the first housing. The battery cell and the voltage conversion circuit are accommodated in an internal space formed by the first housing and the second housing.