To miniaturize a drive coupling mechanism by reducing the required movable amount of a coupling in engaging a drive coupling pair.

In the coupling pair for transmitting drive, at least one coupling element is movable in an axial direction and energized toward the other coupling element forming the pair, and a guide part is provided at an engaging part between the coupling and a shaft so that the coupling element is rotated in the circumferential direction of the shaft when the energized coupling element is pushed into an opposite direction to the energized direction.