To provide a speed reducer in which the inside diameter of a hollow camshaft can be increased without increasing the size.

An output pin 9 can be maintained in rigidity even by narrowing the diameter because both the ends are supported by bearings 10, 11, 15 and 16. Thereby, since a free space can be obtained around the hollow camshaft 1, the diameter of the hollow camshaft 1 can be increased without increasing the size of the speed reduction gear. Since two toothed discs 5 and 6 are provided, when three, for example, toothed discs are provided, a sleeve which is necessary to be fitted into the hollow camshaft 1 can be eliminated. Thereby, without increasing the size of the speed reduction gear, the inside diameter of the hollow camshaft 1 can be increased. In addition, since needle bearings 3 and 4 to rotatably and alternately support the hollow camshaft 1 and the toothed discs 5 and 6 are lower in the radial height than that of a ball bearing, for example, the inside diameter of the hollow camshaft 1 can be increased without increasing the size of the speed reduction gear.