A transmission casing for a bicycle has sufficient strength and rigidity and has a simple structure while being capable of hermetically sealing a transmission body of the bicycle. In a transmission casing for a bicycle is covered with a casing and a plurality of transmission shafts supported by the casing. The transmission casing includes a light-metal-made reinforcing member made of aluminum, a light alloy or the like which supports shaft portions of the plurality of transmission shafts, and a fiber-reinforced-plastic-made cover member which is integrally laminated to an inner surface of the light-metal-made reinforcing member so as to hermetically seal the transmission body. The light-metal-made reinforcing member has a strength necessary for supporting the plurality of transmission shafts, and the fiber-reinforced-plastic-made cover member has a residual rigidity by subtracting rigidity obtained by the metal-made-reinforcing member from the rigidity necessary for preventing the deformation of the whole casing.