A partition member has a thickness direction and a surface direction perpendicular to the thickness direction, and which separates single cells that make up an assembled battery in the thickness direction, or a single cell that makes up the assembled battery in the thickness direction and a member other than the single cells. The partition member includes, in the interior thereof, a fluid having a boiling point at normal pressure of 80° C. to 250° C., and a flow channel of the fluid extending along the surface direction. The fluid is held in a fluid holding part, and the fluid holding part is hermetically sealed by a packaging material.