1. A power supply apparatus comprising: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, the low thermal conductive layer having a thermal conductivity smaller than a thermal conductivity of the first housing,
wherein the battery cell and the voltage conversion circuit are accommodated in an internal space formed integrally by the first housing and the second housing.