A slider (1, 2) of the invention includes a slider body (10, 70), a tab (40), and a cover member (30, 90) that is elastically deformable, the slider body (10, 70) has a front attachment post (21, 81) and a rear attachment post (22, 82) standing up from an upper blade (20, 80), a cover member (30, 90) engages with the front attachment post (21, 81) and the rear attachment post (22, 82), the front attachment post (21, 81) and the rear attachment post (22, 82) have a pair of right and left first side wall portions (21b, 22b, 81b, 82b) and a pair of right and left second side wall portions (21c, 22c, 81c, 82c) arranged on an outside of the first side wall portions (21b, 22b, 81b, 82b) in a length direction, outer wall surfaces of the second side wall portions (21c, 22c, 81c, 82c) are arranged on an inside of the first side wall portions (21b, 22b, 81b, 82b) via step portions (21g, 22g, 81g, 82g) in a width direction, at least a part of the outer wall surfaces of the second side wall portions (21c, 22c, 81c, 82c) faces inner wall surface of the cover member (30, 90). This enables the cover member (30, 90) to be made more difficult to tilt in the width direction even if the cover member (30, 90) receives force in the width direction of the slider.