Abstract: A positioning apparatus for accurately positioning an object such as a work has a guide track secured to the associated frame by a plurality of tightening devices each having a tightening force adjusting member. The guide track is also pressed laterally against a vertical face portion formed on the associated frame by means of a plurality of pressing plates which are affixed to the frame by other tightening devices each having a pressing force adjusting member. Both the tightening devices securely holds the guide track on the frame while allowing relative movement therebetween caused by a difference in the amount of thermal expansion.