The invention provides a shell structure of a transmission, which inhibits the weight increase of a cover of a transmission shell to be in a minimum limit, and effectively inhibits film surface vibration. A cover (54) fixed at an opening part of a transmission body (52) is provided with: a first spherical wall (61) extending outwards in an arch form with an axis (L1) of a first shaft (13) as a center, a second spherical wall (66) extending outwards in an arch form with an axis (L2) of a second shaft (14) as a center, and a third spherical wall (71) which overlaps a part of the first spherical wall (61) and a part of the second spherical wall (66), and extends outwards in an arch form. Therefore, when the invention is compared with a condition in which the first, the second, and the third spherical wall (61, 66, 71) are formed independently of each other, the three spherical walls (61, 66, 71) are mutually enhanced to improve the rigidity of the cover (54); the film surface vibration of the cover (54) can be inhibited effectively without special addition of reinforcing ribs; and the weight increase of the cover (54) is inhibited to be in a minimum limit.