The position of an optical pickup with respect to a disk is maintained accurately by preventing the free end portion of a cantilever movable member from being displaced lower than its proper position during the rotation of the disk even if the movable member is heavy to have improved vibration characteristics. There are provided a movable member 4, a chassis 1, a clamp mechanism 3, and a cam mechanism 5. The cam mechanism 5 includes a shift lever 51 connected to the free end portion of the movable member 4 via elastic bodies 42 and a cam main body 55. Positioning means 8 is composed of a protrusion 53 on the shift lever 51, a stepped engaging surface 15 for supporting the protrusion 53 thereon, and a guide mechanism 71. There is also provided anti-positioning means 9 for disengaging the protrusion 53 from the engaging surface 15.