a case comprising a first main wall, a second main wall facing the first main wall, and an uneven portion comprising a plurality of uneven portions formed in at least one of the first main wall and the second main wall, the plurality of uneven portions comprising a plurality of first uneven portions comprising a convex portion, a concave portion, and a connection portion that are continuously formed in the first main wall such that the convex portion and the concave portion are connected via the connection portion, the convex portion being outwardly convex such that a recess corresponding to the convex portion is formed at an inner surface of the first main wall, the concave portion being outwardly concave such that an embossing corresponding to the concave portion is formed at the inner surface of the first main wall;

an electrode assembly received in an internal space of the case; and

a cap plate sealing an opening at a first end of the case to protect the electrode assembly, the first end being spaced apart from a second end of the case in a longitudinal direction,

wherein the uneven portion is extended from the first end of the case to the second end of the case in the longitudinal direction,

wherein the plurality of first uneven portions further comprises another outwardly convex portion spaced apart from the convex portion in a direction transverse to the longitudinal direction, and

wherein the first main wall and the second main wall have a curvature, and a curvature radius of the second main wall is greater than a curvature radius of the first main wall.