PURPOSE: To regulate the tension of a gut by providing a multiple tuning fork having a plurality of teeth closely to a rounded end part and applying the rounded part against the gut so that one tooth oscillates depending on the tension of the gut.

CONSTITUTION: The inventive apparatus has one elongated end 1 and the other rounded end 2 formed to hit the gut of a racket. The apparatus is provided with a plurality of protrusions 3 on the side face and a multiple tuning fork 5 is provided at the part where the protrusions 3 are provided. A series of marks 7 corresponding to a plurality of teeth 6 of the fork 5, respectively, are put on the protrusions 3 and one tooth 6a of the tuning fork 5 oscillates when the end part 2 touches the gut of racket. Tension of the gut can be determined by detecting oscillation of the tooth 6a and associating the oscillation to the mark 7a.