Abstract: A gear-shifting apparatus is provided which does not require a position of a chain to be set with high precision, and which reduces a risk of the chain coming off an adjacent sprocket due to a quick gear shifting operation. The gear shifting system includes a derailer for changing the chain from one sprocket to another among a plurality of axially-arranged gear shifting sprockets, and a case for containing the gear shifting sprockets and the chain. The case is provided with a protruding part for preventing the chain from moving axially outwardly at an area too close to the case. The protruding part effectively prevents the chain from coming off from the adjacent sprocket by means of contact of the protruding part with the chain.