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, an upper flange protruding downwardly along both left and right edges of the upper blade, a lower flange protruding upwardly 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 inner surfaces of the upper and lower flanges at one side of the body in the width direction are formed as inclined surfaces (59a and 59b) in which thicknesses of the inclined surfaces (59a and 59b) gradually increase as the inclined surfaces (59a and 59b) extend from an inside to an outside in the width direction; and a dimension of a gap between the upper and lower flanges (54a, 54b) in the up-down direction at the one side of the body in the width direction is larger than a dimension of a gap between the upper and lower flanges (54a, 54b) in the up-down direction at another side of the body in the width direction. 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, 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, an upper flange protruding downwardly along both left and right edges of the upper blade, a lower flange protruding upwardly 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 inner surfaces of the upper and lower flanges at one side of the body in the width direction are formed as inclined surfaces (59a and 59b) in which thicknesses of the inclined surfaces (59a and 59b) gradually increase as the inclined surfaces (59a and 59b) extend from an inside to an outside in the width direction; and a dimension of a gap between the upper and lower flanges (54a, 54b) in the up-down direction at the one side of the body in the width direction is larger than a dimension of a gap between the upper and lower flanges (54a, 54b) in the up-down direction at another side of the body in the width direction.