A slider for a concealed slide fastener which engages and disengages fastener elements of a pair of fastener stringers in which opposite side edges of fastener tapes are bent and fixed into U-shapes and the fastener elements are attached along bent end edges of the side edges, the slider comprising: a lower blade; left and right side wall portions (24a, 24b) erected along side edges of the lower blade in a right and left direction; left and right first flanges (25a, 25b) extending from upper ends of the left and right side wall portions (24a, 24b) in a mutually approaching direction; a guide post disposed between the left and right first flanges (25a, 25b) and extending from a central portion of the lower blade in the right and left direction at a shoulder mouths-side toward a rear mouth; a second flange protruding from an upper portion of the guide post toward at least the left and right first flanges (25a, 25b) and the rear mouth; and a pull-tab attaching portion for supporting a pull tab, wherein the slider is comprised of a slider body and a slider upper plate member engaged with the slider body, wherein the slider body has at least the lower blade, the left and right side wall portions (24a, 24b) and the left and right first flanges (25a, 25b), wherein the slider upper plate member has at least the second flange, wherein the guide post is provided in either one of the slider body or the slider upper plate member, and wherein the pull-tab attaching portion is provided in either one of the slider body or the slider upper plate member, characterized in that the second flange, the left and right first flanges (25a, 25b) and inner surfaces (24c, 24d) of the left and right side wall portions (24a, 24b) are formed so as to uniformly increase in widths thereof from a side of the rear mouth to end portions of the shoulder mouths, and outer surfaces of the second flange are substantially parallel to inner surfaces of the left and right first flanges (25a, 25b) such that gaps each having a substantially constant interval are formed between the outer surfaces of the second flange and the inner surfaces of the left and right first flanges (25a, 25b). a lower blade; left and right side wall portions (24a, 24b) erected along side edges of the lower blade in a right and left direction; left and right first flanges (25a, 25b) extending from upper ends of the left and right side wall portions (24a, 24b) in a mutually approaching direction; a guide post disposed between the left and right first flanges (25a, 25b) and extending from a central portion of the lower blade in the right and left direction at a shoulder mouths-side toward a rear mouth; a second flange protruding from an upper portion of the guide post toward at least the left and right first flanges (25a, 25b) and the rear mouth; and a pull-tab attaching portion for supporting a pull tab, wherein the slider is comprised of a slider body and a slider upper plate member engaged with the slider body, wherein the slider body has at least the lower blade, the left and right side wall portions (24a, 24b) and the left and right first flanges (25a, 25b), wherein the slider upper plate member has at least the second flange, wherein the guide post is provided in either one of the slider body or the slider upper plate member, and wherein the pull-tab attaching portion is provided in either one of the slider body or the slider upper plate member, characterized in that the second flange, the left and right first flanges (25a, 25b) and inner surfaces (24c, 24d) of the left and right side wall portions (24a, 24b) are formed so as to uniformly increase in widths thereof from a side of the rear mouth to end portions of the shoulder mouths, and outer surfaces of the second flange are substantially parallel to inner surfaces of the left and right first flanges (25a, 25b) such that gaps each having a substantially constant interval are formed between the outer surfaces of the second flange and the inner surfaces of the left and right first flanges (25a, 25b).