PURPOSE: To perform very efficient and economical measuring work, by providing a crank plate which is coupled with an external operating knob and a plurality of arms, measuring the size between the insides or outsides of two measuring pins protruded from guide grooves, thereby coping the extensive arbitrary sizes in measurement of the inner or outer diameter of a body to be measured.

CONSTITUTION: When the operating knob 4 is rotated, the crank plate 8 is rotated as a unitary body. Five arms 16 attached to the crank plate 8 convert the rotary movement of the crank plate 8 into a linear movement. Five measuring pins 5 attached to the tips of the arms 16 are guided along three guide grooves 18 in a first housing 6 and two guide grooves in a second housing 7 so that the pins are linearly moved. Since the distance between the measuring pin 5 of the arm 16 and a linking pin is the same for all the five pins, the five measuring pins 5 can perform completely equal linear movement in response to the rotary angle of the operating knob 4.