PURPOSE: To move an optical system smoothly by fixing a belt, which is stretched around rollers in both ends of a holding body of a ball, to both of an outside rail and an inside rail in an optical system scanning mechanism where both rails are connected to each other through the ball and the holding body of the ball is moved freely between both rails.

CONSTITUTION: Since an outer member 21 and an inner member 22 are connected to each other through balls 23 in respective curved parts 21a and 22a, the optical system is moved smoothly by rolling friction. Though retainers 24 holding balls 23 are placed between both members and are moved freely, balls 23 are not rotated and are moved by rolling friction if retainers 24 are locked. For the purpose of this movement of balls 23, rollers 27 are provided in both ends of the retainer 24, and a belt 29 is stretched around them, and the belt 29 is fixed to both members, and thus, the retainer 24 is moved forcibly in the half speed of this movement in accordance with the movement.