A slide fastener comprising: a pair of fastener stringers (11L and 11R) provided with fastener element rows having a plurality of fastener elements along opposing tape-side edges of a pair of fastener tapes; and a slider slidably attached to the fastener element rows to engage and disengage the fastener elements, wherein one of the fastener stringers is separated from the slider when a lateral pulling force directed outward in a width direction of the fastener tapes is applied to the pair of fastener stringers, wherein the slider includes: a body having an upper blade and a lower blade which are disposed in parallel while being spaced apart from each other in an up-down direction, a guide post connecting the upper blade and the lower blade at front ends thereof, upper flanges provided along both left and right edges of the upper blade, lower flanges provided along both left and right edges of the lower blade, and a pull tab attachment portion (58F, 58R, 70 and 80) provided on an upper surface of the upper blade; and a pull tab rotatably attached to the pull tab attachment portion. characterized in that the upper blade and the lower blade are provided with cutaway portions (51a and 52a) at portions outward in the width direction from one lateral surface of the guide post, at a side of one of the fastener stringers, and a dimension of a gap between the upper and lower flanges (54a, 54b) at said side of the one of the fastener stringers is set to be larger than a dimension of a gap between the upper and lower flanges (54a, 54b) at a side of the other of the fastener stringers. a pair of fastener stringers (11L and 11R) provided with fastener element rows having a plurality of fastener elements along opposing tape-side edges of a pair of fastener tapes; and a slider slidably attached to the fastener element rows to engage and disengage the fastener elements, wherein one of the fastener stringers is separated from the slider when a lateral pulling force directed outward in a width direction of the fastener tapes is applied to the pair of fastener stringers, wherein the slider includes: a body having an upper blade and a lower blade which are disposed in parallel while being spaced apart from each other in an up-down direction, a guide post connecting the upper blade and the lower blade at front ends thereof, upper flanges provided along both left and right edges of the upper blade, lower flanges provided along both left and right edges of the lower blade, and a pull tab attachment portion (58F, 58R, 70 and 80) provided on an upper surface of the upper blade; and a pull tab rotatably attached to the pull tab attachment portion. a body having an upper blade and a lower blade which are disposed in parallel while being spaced apart from each other in an up-down direction, a guide post connecting the upper blade and the lower blade at front ends thereof, upper flanges provided along both left and right edges of the upper blade, lower flanges provided along both left and right edges of the lower blade, and a pull tab attachment portion (58F, 58R, 70 and 80) provided on an upper surface of the upper blade; and a pull tab rotatably attached to the pull tab attachment portion. characterized in that the upper blade and the lower blade are provided with cutaway portions (51a and 52a) at portions outward in the width direction from one lateral surface of the guide post, at a side of one of the fastener stringers, and a dimension of a gap between the upper and lower flanges (54a, 54b) at said side of the one of the fastener stringers is set to be larger than a dimension of a gap between the upper and lower flanges (54a, 54b) at a side of the other of the fastener stringers.