PURPOSE: To enable continuous absolute measurement of gravity for a long period of time without trouble, by completely eliminating the complicated mechanism in a vacuum cylinder and freely dropping an optical element by utilizing only centrifugal force and gravity.

CONSTITUTION: Vacuum cylinders 1, 7 have a symmetric structure with respect to a rotary shaft A and the rotary shaft A is connected to a motor through a gear and the vacuum cylinders 1, 7 can be rotated around the rotary shaft A to either direction. Receivers 5, 11 are mounted inside both ends of the vacuum cylinders 1, 7. The vacuum cylinders 1, 7 are rotated 180° around the rotary shaft A at such an angular velocity that the centrifugal force acting on the optical element 13 in the receiver 11 does not become smaller than gravity. During the rotation of the vacuum cylinders 1, 7, the element 13 is successively held in the receiver 11 but, the moment the vacuum cylinders 1, 7 are stopped in a vertical direction, centrifugal force is lost and only gravity acts on the element 13 and the element 13 begins to freely fall. By this method, long-term continuous absolute measurement of gravity can be performed without trouble.