To provide a binder with a compact and inexpensive design which prevents a tying string from twisting when binding.

A tubular string roll R is mounted on a rotary table 18 rotationally driven by a motor 19. A diameter inside the roll R for feeding out a tying string S is detected by measuring the weight of the roll R by a load sensor 23 to obtain the number of windings of the string S in the detected feed-out diameter equivalent to a predetermined fed-out length of the string S when binding. The rotary table 18 is driven to rotate in the direction of unrolling the string S by the number of rotations corresponding to the obtained number of windings of the string S. Thus, the string S fed out from the roll R is prevented from twisting from its base, so that the string S can be prevented from twisting when being bound into a compact and inexpensive design.