A speed reducer capable of shortening a length in an axial direction of a turning structure by arranging a plurality of small-sized motors is provided. The hollow speed reducer 10 is a planetary gear apparatus having a crankshaft 11, a pinion 12 formed with outer teeth at an outer periphery thereof, fitted to a crank portion of the crank shaft 11 and eccentrically moved, and a case 13 an inner peripheral face of which is formed with inner teeth brought in mesh with the outer teeth of the pinion 12, in which an inner portion of the hollow speed reducer 10 is formed with a hollow hole, a center gear for an input of the hollow speed reducer 10 is provided at a position of the hollow hole, and small gears provided at output shafts of a plurality of motors 15 are simultaneously brought in mesh with the center gear.