A slide fastener including: a pair of left and right fastener tapes; and a plurality of synthetic resin fastener elements (10, 10') lined along opposing tape side edge portions of the fastener tapes, wherein each fastener element (10, 10') includes an upper half element portion disposed at a first surface side of the fastener tape and a lower half element portion disposed at a second surface side of the fastener tape, the upper half element portion includes a first tape-sandwiching portion having a predetermined dimension in a tape length direction, and a first head portion (13, 13') of a tapered form that extends from the first tape-sandwiching portion toward an outside of the tape up to a forefront of the lower half element portion and has a dimension in the tape length direction that gradually decreases toward the forefront, and the lower half element portion includes a second tape-sandwiching portion that sandwiches the fastener tape together with the first tape-sandwiching portion, a neck that extends from the second tape-sandwiching portion toward the outside of the tape and has a shape constricted in a front and rear of the tape length direction, and a second head portion that extends from a forefront portion of the neck and bulges in a front and rear of the tape length direction; characterized in that: the upper half element portion has a tapered portion in which a dimension between a front side surface and a rear side surface of the first tape-sandwiching portion in the tape length direction is gradually decreased upward; the first head portion (13, 13') includes a first narrow width portion arranged on the first tape-sandwiching portion side and a second narrow width portion arranged on an element forefront side of the first narrow width portion; and when a side edge in the base end portion of the first head portion (13,13') is connected with a side edge in the forefront of the first head portion (13,13') by a straight line, the first narrow width portion is formed to bulge outside the straight line and the second narrow width portion is formed to be recessed inside the straight line. a pair of left and right fastener tapes; and a plurality of synthetic resin fastener elements (10, 10') lined along opposing tape side edge portions of the fastener tapes, wherein each fastener element (10, 10') includes an upper half element portion disposed at a first surface side of the fastener tape and a lower half element portion disposed at a second surface side of the fastener tape, the upper half element portion includes a first tape-sandwiching portion having a predetermined dimension in a tape length direction, and a first head portion (13, 13') of a tapered form that extends from the first tape-sandwiching portion toward an outside of the tape up to a forefront of the lower half element portion and has a dimension in the tape length direction that gradually decreases toward the forefront, and the lower half element portion includes a second tape-sandwiching portion that sandwiches the fastener tape together with the first tape-sandwiching portion, a neck that extends from the second tape-sandwiching portion toward the outside of the tape and has a shape constricted in a front and rear of the tape length direction, and a second head portion that extends from a forefront portion of the neck and bulges in a front and rear of the tape length direction; characterized in that: the upper half element portion has a tapered portion in which a dimension between a front side surface and a rear side surface of the first tape-sandwiching portion in the tape length direction is gradually decreased upward; the first head portion (13, 13') includes a first narrow width portion arranged on the first tape-sandwiching portion side and a second narrow width portion arranged on an element forefront side of the first narrow width portion; and when a side edge in the base end portion of the first head portion (13,13') is connected with a side edge in the forefront of the first head portion (13,13') by a straight line, the first narrow width portion is formed to bulge outside the straight line and the second narrow width portion is formed to be recessed inside the straight line. the upper half element portion has a tapered portion in which a dimension between a front side surface and a rear side surface of the first tape-sandwiching portion in the tape length direction is gradually decreased upward; the first head portion (13, 13') includes a first narrow width portion arranged on the first tape-sandwiching portion side and a second narrow width portion arranged on an element forefront side of the first narrow width portion; and when a side edge in the base end portion of the first head portion (13,13') is connected with a side edge in the forefront of the first head portion (13,13') by a straight line, the first narrow width portion is formed to bulge outside the straight line and the second narrow width portion is formed to be recessed inside the straight line.