An attaching device (10) for attaching a socket member (28) of a snap fastener to a garment has a post (12) having a circular flange (13) extending from a cylindrical stem (12) and a cap (14) mounted thereon. The circular flange (13) has a central portion (17), an intermediate inclined annular step (18), a radially outward annular shoulder (19), and a plurality of radial ribs (20) extending radially inwardly from an end (15) of the stem (12) to the inclined annular step (18). The central portion (17) of the flange (13) has a surface (25) adjacent to the inclined annular step (18) and held against a rounded wall (21) of the cap (14). The end (15) of the post (12) is axially spaced from the rounded wall (21) of the cap (14) with a clearance (27) left therebetween. The annular shoulder (19) of the flange (13) is locked in position by an annular curled rim (24) of the cap (14). Any impact force applied when the attaching device (10) is staked on the socket member (28) is distributed uniformly to the flange (13), preventing the cap (14) from being deformed.