SPEED REDUCER

A speed reducer (1) includes a first transmission shaft (10), an eccentric wheel (11), a first roller assembly (12), a rotating wheel (13), a second roller assembly (14) and a second transmission shaft (15). The rotating wheel (13) includes a main body (130) and an axle hole (133). The main body (130) includes a convex structure (131) and a concave structure (132). The convex structure (131) is protruded from an outer periphery of the main body (130) and has outer teeth. The outer teeth are contacted with the corresponding first rollers (121). The concave structure (131) is concavely formed in a surface of the main body (130) and includes inner teeth. The inner teeth are contacted with plural second rollers (141) of the second roller assembly (14). Since the speed reducer (1) is designed to have four operating situations, the speed reducer (1) can have various reduction ratios.