To determine easily and accurately the hardness of a ball by arranging stopper pins and determination labels at both front and rear ends of a device body made by bending a metal plate into an approximate U shape and varying the angle of opening of the device body in accordance with the hardness of a tennis ball inserted into the interior of the device body.

A measuring instrument 1 is formed by bending a metal plate into an approximate U shape. A stopper pin 2 is penetrated into the front end of the measuring instrument 1 and a determination label 3 marked in a plurality of colors including white, red, yellow and blue is pasted to the rear end and a determination window 4 is opened at the rear end face. In using, a tennis ball 5 is inserted into the interior of the measuring instrument 1, and the end is contacted to the stopper pin 2. Then, in accordance with the hardness of the ball, the angle of opening of the measuring instrument 1 varies, and the color of the determination label 3 visible from the determination window changes. Therefore, by determining the color seen, the hardness of the ball can be determined easily and accurately.