A speed reducer, comprising: a first transmission shaft having a first end and a second end; an eccentric wheel eccentrically fixed on the second end of the first transmission shaft; a first roller assembly comprising a first wheel disc and plural first rollers, wherein the first wheel disc is arranged between the first end and the second end of the first transmission shaft, the plural first rollers are disposed on the first wheel disc, and the plural first rollers are selectively self-rotated; a rotating wheel comprising a main body and an axle hole, wherein the eccentric wheel is rotatably disposed within the axle hole, and the main body comprises a convex structure and a concave structure, wherein the convex structure is protruded from an outer periphery of the main body and has one or plural outer teeth, and the outer periphery of the main body is contacted with the corresponding first rollers, wherein the concave structure is concavely formed in a surface of the main body and comprises one or plural inner teeth; a second roller assembly comprising a second wheel disc and plural second rollers, wherein the plural second rollers are disposed on the second wheel disc, the plural second rollers are selectively self-rotated, and the second rollers are contacted with the corresponding inner teeth; and a second transmission shaft having a third end and a fourth end, wherein the second wheel disc is fixed on the third end of the second transmission shaft, wherein the number of the outer teeth and the number of the inner teeth are different, the number of the first rollers is at least one more than the number of the outer teeth, and the number of the second rollers is at least one more than the number of the inner teeth. a first transmission shaft having a first end and a second end; an eccentric wheel eccentrically fixed on the second end of the first transmission shaft; a first roller assembly comprising a first wheel disc and plural first rollers, wherein the first wheel disc is arranged between the first end and the second end of the first transmission shaft, the plural first rollers are disposed on the first wheel disc, and the plural first rollers are selectively self-rotated; a rotating wheel comprising a main body and an axle hole, wherein the eccentric wheel is rotatably disposed within the axle hole, and the main body comprises a convex structure and a concave structure, wherein the convex structure is protruded from an outer periphery of the main body and has one or plural outer teeth, and the outer periphery of the main body is contacted with the corresponding first rollers, wherein the concave structure is concavely formed in a surface of the main body and comprises one or plural inner teeth; a second roller assembly comprising a second wheel disc and plural second rollers, wherein the plural second rollers are disposed on the second wheel disc, the plural second rollers are selectively self-rotated, and the second rollers are contacted with the corresponding inner teeth; and a second transmission shaft having a third end and a fourth end, wherein the second wheel disc is fixed on the third end of the second transmission shaft, wherein the number of the outer teeth and the number of the inner teeth are different, the number of the first rollers is at least one more than the number of the outer teeth, and the number of the second rollers is at least one more than the number of the inner teeth.