An eccentrically oscillating speed reducer 100 includes a case 110 having inner teeth 110a, a plurality of crankshafts having outer teeth, outer teeth gears 131 and 132 having outer teeth 131a and 132a brought in mesh with the inner teeth 110a of the case 110 and eccentrically moved by the crankshafts, a carrier 140 rotatably supporting the crankshafts and rotated to move relative to the case 110 by the crankshafts, and a cylindrical gear 180 brought in mesh with the outer teeth of the crankshafts for transmitting a power inputted to one crankshaft 121 of the plurality of crankshafts to remaining crankshafts, and the cylindrical gear 180 is rotatably supported by sliding friction.