34 in the upper surface of the slider 13 this retractive movement of the slider 13 causes the ledge 31 to disengage from the hook 25 to reveal the gap 33 therebetween and simultaneously the resilient strips 28 28 to elastically spread against their tension within the wide portions 27a 27a of the slits 27 27 as the engaging ribs 32 32 slide back with their guide surfaces 32 39 32 39 held in contact with the prongs 28 39 28 39 of the resilient strips 28 28 as shown in fig7 releasing the finger pressure will allow the resilient strips 28 28 to restore to the original position by their own resiliency whereby the engaging ribs 32 32 are biased to move forward and snap into the narrow portions 27b 27b of the slits 27 27 until the gap 33 is closed as shown in fig5 and 6 in the above reciprocating movement of the slider 13 relative to the base 11 it is to a great advantage that a pull force required to retract the slider 13 is very small compared to the prior art discussed at the outset of this specification and that the provision of alternate ridge and groove engagement 25a 25b 25b and 31a 31b 31b of the ledge 31 with the hook 25 ensures freedom of deformation of these parts when subjected to torsional stresses reference to fig9 and 10 shows a modified form of strap fastener 10 39 according to the invention this modified strap fastener 10 39 differs from the first advanced strap fastener 10 in that the base member 11 is made rotatable relative to the retainer member 12 so that the strap s wound on the cross bar 21 can be.