surfaces 35b of the needle thread loops 35 are raised above the surfaces 36c of the looper thread 28 and such required degrees of tension f f for the looper thread 28 and the needle thread 27 are defined by the following inequalities as shown in fig7 the individual needle thread loop 35 has a widened portion 35c after stitching of the fastener elements 26 to the stringer tape 24 25 as described above the needle thread 27 which consists of a thermoplastic monofilament yarn has been heat set by applying a heated medium for instance during a dyeing process as a result the needle thread 27 has become dimensionally stable that is the bent configurations 35a 38 39 fig3 and 4 of the needle thread 27 are maintained against further dimensional change thereby preventing the needle thread 27 as well as the looper thread 28 from becoming loose at the cut end portions 40 41 of the slide fastener stringers 22 23 respectively more specifically in case the needle thread 27 has been cut such that its cut end 42 point upwardly fig4 the needle thread 27 is not likely to become loose because the bent configuration 38 of the needle thread 27 is kept in stable in case the needle thread 17 has been cut such that its cut end 43 point downwardly fig3 the looper thread 28 is held at its cut end portion by the extreme needle thread loop 35 of which bent configuration.