Slide fastener

In slide fasteners (10, 40, 50), butterfly sticks (31, 51) have portions (31b, 51b) stretching toward box sticks (32, 32', 52), and the difference between the interval A between the inner surfaces of right and left flanges (24) at the rear ends of sliders (21a, 21b) and the maximum value B of dimension of the butterfly sticks (31, 51) in the tape width direction (interval A minus maximum value B) is set to satisfy the relation "the dimension C in the tape width direction at a part of the box sticks (32, 32', 52) corresponding to a part of the butterfly sticks (31, 51) having the maximum value B < (the interval A minus the maximum value B)(the length of three pitches of the fastener element (15)) minus (the dimension in the tape length direction at a meshing head (15a))3". Since the right and left element lines (13) are not meshed intentionally even if the slider (21a) is slid under a state where the butterfly sticks (31, 51) are inserted insufficiently, fall in meshing strength or weakening in meshing strengt