To provide a cooling or heating structure of a battery that is easy to be assembled.

A battery unit 100 includes a plurality of batteries 10, and heat exchanger plates 20 arranged between the batteries 10. The heat exchanger plates 20 have: respective battery-facing portions 22 that face the batteries 10; and respective extension portions 24 that extend outward from the respective battery-facing portions 22 and beyond outer edges of the batteries 10. The extension portions 24 transfer external heat to the batteries 10 through the respective battery-facing portions 22, or dissipate heat received from the batteries 10 through the respective battery-facing portions 22, to the outside.