The diameter of a front end opening section (20-1) extending rearward over a predetermined length from a front end opening of a male member reception hole (20) is larger than a diameter of a male member reception section (20-2) of the male member reception hole that follows the front end opening section. The backup ring (18) is allowed to engage an inner wall surface (20-3) of a front end opening section (20-1) as a backup ring (18) is pressed radially outward from an annular groove (24) by a seal ring (16) that is deformed by engagement with the inner wall surface of the male member reception hole (20) when a male coupling member (12) begins to be inserted into the male member reception hole (20), which prevents the backup ring (18) from coming out, in a radially outward manner, from the annular groove (24).