To easily spin a ball, to prevent the rubbing of strings and to improve a durability by continuously interleaving a thick width and a thin width in the strings from the point of side-face view.

In the strings A extended on the phase surfaces of various kinds of rackets, it is favorable that the thick width 2a and the thin width 2b are continuously and regularly interleaved and the pitch of the strings A is 20-100 mm, especially, 25-30 mm. When the pitch is too small or too large, a spin amount to be given to the ball is hardly changed. Then, it is favorable that the thick width 2a is within the range of 1.35-1.6 mm and the thin width 2b is 0.8-1.35 mm and, moreover, 1.20-1.35 mm is favorable when a configuration consists of only one core string 1a. When the cross section of the strings A is an oval shape, 30-200 mm/time twisting pitch is favorable. Besides, the thickness of the strings A is preferably 0.8-1.6 mm, most favorably 1.2-1.4 mm.