A battery unit includes a box-shaped case in which a plurality of secondary batteries are stored and that includes a front face, a back face, a first lateral face, a second lateral face, a first main face, and a second main face, a first heat-transfer face that is provided on one faces of the first and second main faces of the case, a second heat-transfer face that is formed on at least one faces of the first and second lateral faces and is continued to the first heat-transfer face, and an insulating face that is formed on the front face, the back face, the other faces of the first and second main faces, and an inner face of the second heat-transfer face. In the battery unit, a battery element of the secondary batteries is stored in an outer package member and positive and negative electrode tabs are led out.