A speed reducer includes a first transmission shaft, an eccentric wheel, a first roller assembly, a rotating wheel, a second roller assembly and a second transmission shaft. The rotating wheel includes a main body and an axle hole. The main body includes a convex structure and a concave structure. The convex structure is protruded from an outer periphery of the main body and has outer teeth. The outer teeth are contacted with the corresponding first rollers. The concave structure is concavely formed in a surface of the main body and includes inner teeth. The inner teeth are contacted with plural second rollers of the second roller assembly. The speed reducer of the present invention is an epicycloid-epicycloid speed reducer or a hypocycloid-hypocycloid speed reducer. Since the speed reducer is designed to have four operating situations, the speed reducer can have various reduction ratios.