Disclosed is a structure of a casing of a small stepping motor having an outer diameter of 8 mm or less, in which yoke teeth are integrally formed on the inner surface of the casing, and the thickness of the outer wall of the casing is different from the thickness of each of the yoke teeth. The casing of a small stepping motor includes yoke teeth integrally formed on an inner surface of the casing, in which a thickness of an outer wall of the casing is different from a thickness of the yoke tooth.