PURPOSE: To permit measurement of the relative displacements between optional two members which are coupled displaceably by a rotary joint without need for any special devices and immobile points by determining said displacements from the relative angular displacements between the two members.

CONSTITUTION: The fine extent L3 of movement of members 28 which are coupled by means of members 21a, 21b via a rotary joint 21 is determined by calculation from the displacement θ1-θ2 of the relative angle between the members 21a and 21b. Then desired displacements are measured easily without using any immobile points and devices such as measuring instruments as in the prior art.