PURPOSE: To measure the inside perimeter length anywhere at the bead of a tire highly precisely and quickly by providing an encoder to generate pulses corresponding to the rotating speed of a roller.

CONSTITUTION: A tire 30 to be measured is laid in the state that it is turned down on a level floor. A measuring instrument 10 is set up inside the tire. and a roller 17 supported by a roller supporting lever 14 is put in press contact with the portion to be measured between the bead toe on the inner periphery of a tire bead 31 and a rim cushion via a damper 21. In such a state, the roller supporting lever 14 is driven and rotated one turn around the axial center of the tire 30. With the rotation of the roller supporting lever 14, the roller 17 put in contact with the inner periphery of the tire bead 31 is given one turn while being rotated and set back to an original contact position. The number of pulses of an encoder 25 which is generated corresponding to the rotating speed of the roller 17 is read thereto obtain the inside perimeter of the tire bead 31 which is found from a relationship between a preset unit pulse number and the outside perimeter of the roller 17.