PURPOSE: To provide a ring gauge measuring apparatus which allows the measuring of not only diameter but also roundness by recording and comparing the measured value of the inner circumference of the reference ring gauge and of the ring gauge laid one upon another on the same circular recording paper.

CONSTITUTION: A coordinate table 12 is mounted on the upper portion of a table rotation mechanism 2, serving as the rotary table 6 and a positioning mechanism. A probe 27 vertically movable having a detector 30 at the lower end thereof is provided on a carriage 24 slidable in the other direction. Then, the reference ring gauge 18 and the ring gauge 19 to be measured are placed on the table 12. The table 12 is adjusted by turning the table 6 so that the swing of the measuring pointer 43 reaches its minimum whereby both the gauges are centered. Under such a condition, the probe 27 is lowered to measure the inner diameter at a given point initially on the gauge 18 and then the gauge 19 and the measured valves are recorded on a circular recording paper at the recording section 34 laid one upon another. Thus, comparison of the circular diagram allows the measurement of the diameter and the roundness.