POWER SOURCE APPARATUS

The purpose of the present invention is to keep the temperature balance between a battery cell and a control element in an optimum range, and to prevent a deterioration of electric characteristics due to a battery cell temperature increase and an operation failure due to a control element temperature increase. This power supply device includes a battery assembly (40) having battery cells (1) and a circuit substrate (80) on which a control element (82) for providing a protection circuit for the battery cells (1) is mounted, the circuit substrate (80) being fixed to a substrate holder (81). A bottom plate (81A) of the substrate holder (81) is disposed between the circuit substrate (80) and the battery assembly (40). The control element (82) is fixed to a surface of the circuit substrate (80) and is closely contacted with a potting resin (7). A heat insulating layer (83) is disposed between a back surface of the circuit substrate (80) and the bottom plate (81A).