A battery pack comprising: a first unit battery including: a wound electrode body in which a winding axis extends along a second direction that is orthogonal to a first direction; and a casing that accommodates the wound electrode body and an electrolyte solution; a second unit battery that opposes the first unit battery in the first direction; and a spacer (3; 3A; 3B; 3C) that is arranged between the first unit battery and the second unit battery, the spacer (3; 3A; 3B; 3C) including: a primary surface that faces the first unit battery; and plural projections (35, 36), each of which is projected from the primary surface toward the first unit battery side, wherein the plural projections (35, 36) include: a first projection that abuts the casing in a state where the casing is not expanded; and a second projection, a height of which is lower than a height of the first projection in the first direction, the second projection is configured to abut the casing so as to alleviate stress applied to the first projection in a state where the casing is expanded for a specified volume, the first projection includes a first rib and a second rib, the first rib is at least in each of paired first regions of the primary surface, the paired first regions being regions that respectively overlap ends of the wound electrode body in the second direction when seen in the first direction, and the second projection is at least in a second region of the primary surface, the second region being a region located between the paired first regions, characterized in that the second projection (36, 361) is on one side of the spacer (3; 3A; 3B; 3C) in a third direction that is orthogonal to the first direction and the second direction, and in that a portion of the second rib of the first projection is on the other side of the spacer (3; 3A; 3B; 3C) in the third direction and extends from one side to the other side in the second direction. a first unit battery including: a wound electrode body in which a winding axis extends along a second direction that is orthogonal to a first direction; and a casing that accommodates the wound electrode body and an electrolyte solution; a second unit battery that opposes the first unit battery in the first direction; and a spacer (3; 3A; 3B; 3C) that is arranged between the first unit battery and the second unit battery, the spacer (3; 3A; 3B; 3C) including: a primary surface that faces the first unit battery; and plural projections (35, 36), each of which is projected from the primary surface toward the first unit battery side, wherein the plural projections (35, 36) include: a first projection that abuts the casing in a state where the casing is not expanded; and a second projection, a height of which is lower than a height of the first projection in the first direction, the second projection is configured to abut the casing so as to alleviate stress applied to the first projection in a state where the casing is expanded for a specified volume, the first projection includes a first rib and a second rib, the first rib is at least in each of paired first regions of the primary surface, the paired first regions being regions that respectively overlap ends of the wound electrode body in the second direction when seen in the first direction, and the second projection is at least in a second region of the primary surface, the second region being a region located between the paired first regions, characterized in that the second projection (36, 361) is on one side of the spacer (3; 3A; 3B; 3C) in a third direction that is orthogonal to the first direction and the second direction, and in that a portion of the second rib of the first projection is on the other side of the spacer (3; 3A; 3B; 3C) in the third direction and extends from one side to the other side in the second direction.