To provide a speed reducer improved in a withstand load performance.

This speed change gear comprises: an outer roller 10 having, at an outer periphery, outer roller grooves 10A by the first number of repetition; a guide roller 20 combined with the outer periphery side of the outer roller and having guide roller grooves 20A by the second number of repetition; an inner roller 30 combined with the outer periphery side of the outer roller to be inwardly covering the guide roller grooves and having, at an inner diameter side, inner roller grooves 30A extending in an axial direction at intervals in a peripheral direction; and a plurality of balls 40 each of which is arranged at a position where each of the outer roller grooves and each of the guide roller grooves intersects, so as to be capable of rocking inside each of the inner roller grooves. Especially, the guide roller is constituted of two rotors 20-1 and 20-2 divided with the guide roller groove as a boundary. The two rotors have cylindrical bodies 23-1 and 23-2 extending so as to be opposite to each other, and the guide roller groove is formed between tips of the cylindrical bodies 23-1 and 23-2.