PURPOSE: To position a point to be measured at the same position regardless of the size of a material to be measure by setting a ratio of distances from a fixed shaft to a first and a second connecting points equal to a ratio of a moving quantity of a pair of pinching members.

CONSTITUTION: A material 3 to be measured has a point P to be measured at an inside position parted from any point of the peripheral surface with the same ratio. A first and a second pinching members 25, 26 are held by slide struts 27, 28 freely to go forward and backward, and are coupled with a turning lever 42 at a constant ratio. That turning lever 42 is turnably supported by a fixed shaft 43, and has a first and a second connecting points 44, 45, and when the tuning lever 42 is turned clockwise, a pair of pinching members 25, 26 come close to each other for pinching the material 3 to be measured. A ratio of a distance L1 from the fixed shaft 43 to the first connecting point 44 and a distance L2 from the fixed shaft 43 to the second connecting point 45 is set so as to be the same with a ratio of displacement quantities d1, d2 of the pinching members 25, 26. A point P to be measured of a material 3 to be measured in various sizes can be positioned at a specified position.