To provide a battery pack capable of preventing the entire output of battery pack from dropping, which is caused by a temperature difference between batteries constituting the battery pack.

A plurality of batteries are connected in parallel or in series in a battery pack. A protrusion/recess part having at least one of protrusion/ recess shapes among nearly mound, nearly rectangular solid, nearly trapezoid, nearly semispherical shape, and nearly hole shape, is provided to the connection part between the batteries.