A mechanism deck provided with a turntable for rotating a disk and movable upward or downward as coupled to a slide member comprises a subchassis having the turntable mounted thereon, and a frame fitting to the slide member and supporting the subchassis. The chassis is provided with a holding mechanism for upwardly or downwardly pivotally movably supporting the frame as fitted to the slide member through a space between the clamp support plate and the chassis. The frame is provided with a pin projecting outward therefrom and supported by the holding mechanism for restraining the frame from backlashing upward or downward, and the projecting pin has a projection for preventing the frame from backlashing laterally.