To simply form a rubber-ring socket not causing an O-ring to roll over.

In this rubber-ring socket 10, the O-ring 20 is held between projections 18a, 18b inwardly projecting from an inner wall of a socket 14. In forming the socket 10, the O-ring 20 is mounted on a rubber-ring mount part 36 of a metallic pattern 22, and the pattern 22 is inserted into an end part of a pipe 14a softened by heating. By driving a vacuum pump, a negative pressure is given to a rubber-ring suction hole 44 and to suction holes 46a, 46b, thereby causing the O-ring 20 to be sucked to the mount part 36 and the socket 14 to be inwardly sucked to form the projections 18a, 18b.