A battery module configured to be joined to a joint body, the battery module comprising: a plurality of battery cells; and a plurality of heat transfer plates, each of which is located between adjacent ones of the battery cells, wherein: each heat transfer plate includes a sandwiched portion, which is located between adjacent ones of the battery cells, and a joined portion, which is configured to be joined to the joint body and includes a facing surface that faces the joint body; a first heat conductive layer that is in contact with at least one of the battery cells is located on the sandwiched portion of each heat transfer plate, wherein the first heat conductive layer has a first thickness between a part that is in contact with the sandwiched portion and a part that is in contact with the battery cell; a second heat conductive layer is located on the facing surface of each of the heat transfer plates or on the facing surface of each of the heat transfer plates except for one of the heat transfer plates that is the closest to the joint body when the joined portions are joined to the joint body, wherein the second heat conductive layer is configured to contact the joint body; and the second heat conductive layer on one of the heat transfer plates that is the farthest from the joint body when the joined portions are joined to the joint body has a second thickness between a part that is in contact with the joined portion and a part that is in contact with the joint body, and the second thickness is greater than the first thickness. a plurality of battery cells; and a plurality of heat transfer plates, each of which is located between adjacent ones of the battery cells, wherein: each heat transfer plate includes a sandwiched portion, which is located between adjacent ones of the battery cells, and a joined portion, which is configured to be joined to the joint body and includes a facing surface that faces the joint body; a first heat conductive layer that is in contact with at least one of the battery cells is located on the sandwiched portion of each heat transfer plate, wherein the first heat conductive layer has a first thickness between a part that is in contact with the sandwiched portion and a part that is in contact with the battery cell; a second heat conductive layer is located on the facing surface of each of the heat transfer plates or on the facing surface of each of the heat transfer plates except for one of the heat transfer plates that is the closest to the joint body when the joined portions are joined to the joint body, wherein the second heat conductive layer is configured to contact the joint body; and the second heat conductive layer on one of the heat transfer plates that is the farthest from the joint body when the joined portions are joined to the joint body has a second thickness between a part that is in contact with the joined portion and a part that is in contact with the joint body, and the second thickness is greater than the first thickness. each heat transfer plate includes a sandwiched portion, which is located between adjacent ones of the battery cells, and a joined portion, which is configured to be joined to the joint body and includes a facing surface that faces the joint body; a first heat conductive layer that is in contact with at least one of the battery cells is located on the sandwiched portion of each heat transfer plate, wherein the first heat conductive layer has a first thickness between a part that is in contact with the sandwiched portion and a part that is in contact with the battery cell; a second heat conductive layer is located on the facing surface of each of the heat transfer plates or on the facing surface of each of the heat transfer plates except for one of the heat transfer plates that is the closest to the joint body when the joined portions are joined to the joint body, wherein the second heat conductive layer is configured to contact the joint body; and the second heat conductive layer on one of the heat transfer plates that is the farthest from the joint body when the joined portions are joined to the joint body has a second thickness between a part that is in contact with the joined portion and a part that is in contact with the joint body, and the second thickness is greater than the first thickness.