PURPOSE: To reduce the measuring error of a measuring instrument in the Z-axis direction of a supporting arm and to maintain the instrument at a fixed height even when the instrument is used for measurement for a long time by offsetting the upward linear expansion of the supporting arm by making an auxiliary arm to make linear expansion downward.

CONSTITUTION: The coefficients of linear expansion of a supporting and auxiliary arms 2 and 3 are respectively set at (α1 and α2 (α1>α2). The length L1 from the center of the pin 8 of the arm 2 to a pedestal 1 and length L2 from the center of the pin 8 to a holding section 10 are decided depending upon the coefficients α1 and α2 so that a relation L /L =C42/Ql can be met. When the temperature rises by t°C, the arm 2 extends upward by, ΔL1=L1.α1.t and the arm 3 extends downward by ΔL2=L2.α2.t. From the abovementioned equations, ΔL1=ΔL2 holds and the upward extension of the arm 2 is offset by the downward extension of the arm 3. Therefore, the height of the holding section 10 does not vary and a measuring instrument 11 can accurately measure an object to be measured.