In a method of and apparatus for forming element-free spaces in a continuous slide fastener chain at ends of successive fly strips sewn to the slide fastener chain, the chain having a pair of intermeshed rows of coupling elements, portions of the intermeshed rows of coupling elements to be removed are detected by a sensor assembly disposed at a fixed position spaced a first distance upstream from a gap-forming device before such portions arrive at the gap-forming device. The sensor assembly first determines whether the longitudinal extent of a space provided between the confronting ends of an adjacent pair of the fly strips is larger than a predetermined distance, and then on the bases of the determination, controls the feeding of the chain and the operation of the gap-forming device so that the element-free spaces can be provided accurately at the ends of the respective fly strips regardless of the spacing between adjacent fly strips.