Abstract: A speed-reducing or -increasing apparatus, with first and second crown gears facing one another, with a cam unit that causes the first crown gear to incline with respect to the second crown gear so that the first crown gear meshes with the second crown gear, that causes the first crown gear to precess in such a manner as to move the meshing position, and couples to an input or output shaft. A rolling element is between the cam unit and the first crown gear, and the cam unit includes a first member that couples to the input or output shaft and a second member made of steel and including a rolling element rolling portion where the rolling element rolls, the second member configured to be incapable of rotating relatively to the first member, and at least part of the first member having a lower specific gravity than the second member.

Abstract: Provided is a spline structure with which contact surface area between an outer-side member and balls can be ensured, and contact surface area between an inner-side member and the balls can be ensured. This spline structure is provided with an outer-side member having a first track groove on the inner surface, an inner-side member having a second track groove on the outer surface, and balls disposed between the first track groove and the second track groove so as to allow the inner-side member to be inclined relative to the outer-side member. A bottom part extending along the length direction of the first track groove has a linear shape. A bottom part extending along the length direction of the second track groove has a concave curved shape.