Deceleration mechanism

A deceleration mechanism includes: an input axle having a wave generation part with a non-cylindrical shape, the wave generation part having a curved surface formed by a continuous segmented cylindrical surface; a bearing ring having a plurality of bearing holes, each arranged therein with a running pulley moveable with respect to the bearing ring; and a socket ring having an inner surface formed thereon with a plurality of sockets, each capable of receiving at least a part of the running pulley so that the socket and the running pulley can apply force to each other. The wave generation part of the input axle, the bearing ring, and the socket ring are coaxially arranged in sequence from inside to outside. When all of the running pulleys are abutted against the wave generation part, at least one of the running pulleys is furthest received in one of the sockets, and at least another one of the running pulleys is not received in any one of the sockets.