PURPOSE: To provide a shaft coupling whose shaft directional length is shortened by arranging at least either one of the first shaft joining part and the second shaft joining part coaxially in layers upon an elastic body.

CONSTITUTION: A collar 25 inserted in a motor side detector joining shaft 3 is composed of a simple shape part to which inside diameter work of a round bar material and key groove work are applied, and is arranged in layers upon an inside diameter part of a bellows 9, and is constituted so that the motor side detector joining shaft 3 is inserted up to a shaft directional inside part of a shaft coupling 20. In order to prevent the shaft directional movement of the motor side detector joining shaft 3 and the collar 25, a locking screw 13 is arranged in one place or in two places in the circumferential direction. Thereby, the motor side detector joining shaft 3 can be formed in the shape of being arranged in layers on the inside diameter side of the bellows 9, so that a shaft directional length is shortened. That is, an overall length of a motor having a rotary detector can be shortened.