this invention provides an engaging member of a fastening device integrally molded of synthetic resin which enables sewing thereof to clothes easily and accurately , ensures ease and smoothness in an operation for opening the engaging member of the fastening device even if it is molded relatively thick , and prevents too much force being applied to a sewing yarn by an external force upon the opening operation thereby ensuring durability and contributes to reduction of production cost . the engaging member of the fastening device is integrally molded with a substantially j - shaped section , consisting first and second flat plate portions substantially parallel with different lengths and made of thermoplastic synthetic resin material , whose each of end portions is connected via a connecting portion , wherein a number of engaging elements are formed on the surface of the first flat plate portion having a larger area such that they are in an upright position and a groove - like sewing line is formed along an entire peripheral edge portion thereof , a groove - like sewing line whose end portions are open is formed corresponding to part of the sewing line formed on the first flat plate portion on the second flat plate portion having a smaller area , and a tab made of the same resin material is molded integrally on the connecting portion so as to project outward from the connecting portion in parallel to the flat plate portions .