PURPOSE: To obviate personal differences in measurement by setting a measure made of a flat spring, wound to the outside periphery of a ball, to a prescribed tension by utilizing the gravity of a weight fixed to the measure through another flat spring, and measuring the length of the outside periphery of the ball.

CONSTITUTION: A post 3 which has been hollowed out by the same circular arc as the outside periphery of a ball is fixed onto a base D. A measure 2 made of a flat spring is wound in parallel to the base D, and formed to a flexible loop by making its movable end pass through a groove E which has been cut in the post 3. The measure 2 made of a flat spring is made to go along the maximum outside periphery by putting the ball 1 into the loop and drawing the movable end of the measure made of a flat spring by a prescribed force. The movable end of the measure 2 made of a flat spring is pushed to a prescribed position and a position of the movable end at the tine when the loop has been opened is set to a reference position G, and from its reference position G, a distance (x) by which the movable end has moved is measured, and length of the outside periphery of the ball 1 is calculated.