To provide a mounting structure for an air cylinder positioner which is never projected outside in the cross direction of the air cylinder.

This structure for mounting a positioner for air cylinder has a position detecting unit for detecting a rod 23 of an air cylinder 21 so as to convert it to the angle of rotation and an air proportional valve for controlling the driving air to the air cylinder 21, and feedback of the angle of rotation, which is detected by the position detecting unit, to the air proportional valve is performed so as to control the rod 23 at a position corresponding to the input signal. This structure is also provided with a slide plate cam 12, of which one end is fitted to the rod 23 of the air cylinder 21 and which is formed with a groove cam 13 in the sliding direction of the rod 23, and a follower pin for following the groove cam 13 of the slide plate cam 12, and a rotating lever for converting the position of the follower pin to the angle of rotation.