PURPOSE: To open and close bows with adequate tightness by forming the pivot part of each bow by fitting a hemispheric or elliptic projection part in a recess part, and by providing the upper and lower pieces of it with elasticity.

CONSTITUTION: The pivot part of each bow is formed by fitting a hemispherical or ellipstic projection part in a recess part, and a deep cut 12 is made in a fitting-in part 9 at the tip of the bow to divide it into two by the cut 12; and since vertical elasticity is provided where a projection or recess part is made, both upper and lower pieces are opened slightly to fit the recess part of the fitting-in part 9 of the bow around each projection part of the upper piece 2 and lower piece 3, and the projection part of the fitting-in part of the bow in each recess part of the upper and lower piece. Then, the upper and lower pieces are tightened up with a screw 14 to brough the upper and lower pieces and the fitted-in projection and recess parts of the fitting-in part of the bow into close contact by the elasticity of the fitting-in part. Thus, the bows are opened and closed with adequate tightness.