To provide a shaft motion detection mechanism capable of detecting the displacement of a rotating body in a direction along a rotational axis and whether or not the rotating body is rotated, and a conditioner head capable of detecting the displacement of a polishing disk in a direction along a rotational axis and whether or not the polishing disk is rotated. A conditioner head 10 has a shaft 18 rotating around a rotational axis C, a cylindrical shaft 19 attached to the shaft 18 so as to be displaced along the rotational axis C, a frame 16 rotatably supporting the shaft 19, a plurality of projections 18c formed on the surface of the shaft 18 so as to be aligned in a circumferential direction, a projection 18d formed throughout the entire circumference on the surface of the shaft 18, and proximity sensors 30a to 30c arranged around the rotational axis C so as to face the shaft 18. The angle formed around the rotational axis C by the proximity sensors 30a and 30c is larger than the angle formed around the rotational axis C by the circumferential width of each projection 18c and the angle formed around the rotational axis C by the space between the projections 18c.