A coupling member (22, 24) of a pipe coupling having a male coupling member and a female coupling member that are couplable to each other; said coupling member (22, 24) comprising: a tubular member (32, 42) having a fluid passage (30, 40), said tubular member having an annular valve seat surface formed so as to surround said fluid passage (30, 40); and an on-off valve (26, 46) provided in the fluid passage of said tubular member, said on-off valve being movable between a closed position where it is engaged under pressure with said annular valve seat surface to close said fluid passage (30, 40) and an open position where said on-off valve (26, 46) is displaced rearward in an axial direction of said fluid passage to separate from said annular valve seat surface, thereby opening said fluid passage (30, 40); said on-off valve (26, 46) comprising: an annular valve surface facing said annular valve seat surface, the annular valve surface having an annular groove formed in the annular valve surface, an annular forward valve surface portion located forward of said annular groove and an annular rear valve surface portion located rearward of said annular groove, said annular groove defined between said annular forward and rearward valve surfaces; and a seal ring fitted and fixed in said annular groove; wherein when said on-off valve is in the closed position for closing said fluid passage (30, 40), said seal ring is sealingly engaged under pressure with said annular valve seat surface; said annular rear valve surface portion having a rear annular passage limiting portion adapted to be in contact with or close proximity to said annular valve seat surface, wherein said rear annular passage limiting portion is moved while keeping in contact with or close proximity to said annular valve seat surface during a period from when said on-off valve starts to move from said closed position toward said open position until said seal ring has separated from said annular valve seat surface by a predetermined distance; said coupling member being characterized in that: said annular groove has a forward annular groove portion and a rear annular groove portion shallower than said forward annular groove portion; said annular rear valve surface portion of said annular valve surface has a larger radius than that of said annular forward valve surface portion; said seal ring has: a forward annular seal portion fitted in said forward annular groove portion and having a larger radius than that of the annular forward valve surface portion of said annular valve surface; and a rear annular seal portion contiguous with said forward annular seal portion and having an outer diameter that is substantially same as that of said forward annular seal portion but smaller than that of said annular rear valve surface portion, said rear annular seal portion being fitted and fixed in said rear annular groove portion. a tubular member (32, 42) having a fluid passage (30, 40), said tubular member having an annular valve seat surface formed so as to surround said fluid passage (30, 40); and an on-off valve (26, 46) provided in the fluid passage of said tubular member, said on-off valve being movable between a closed position where it is engaged under pressure with said annular valve seat surface to close said fluid passage (30, 40) and an open position where said on-off valve (26, 46) is displaced rearward in an axial direction of said fluid passage to separate from said annular valve seat surface, thereby opening said fluid passage (30, 40); an annular valve surface facing said annular valve seat surface, the annular valve surface having an annular groove formed in the annular valve surface, an annular forward valve surface portion located forward of said annular groove and an annular rear valve surface portion located rearward of said annular groove, said annular groove defined between said annular forward and rearward valve surfaces; and a seal ring fitted and fixed in said annular groove; wherein when said on-off valve is in the closed position for closing said fluid passage (30, 40), said seal ring is sealingly engaged under pressure with said annular valve seat surface; said annular rear valve surface portion having a rear annular passage limiting portion adapted to be in contact with or close proximity to said annular valve seat surface, wherein said rear annular passage limiting portion is moved while keeping in contact with or close proximity to said annular valve seat surface during a period from when said on-off valve starts to move from said closed position toward said open position until said seal ring has separated from said annular valve seat surface by a predetermined distance; said annular groove has a forward annular groove portion and a rear annular groove portion shallower than said forward annular groove portion; said annular rear valve surface portion of said annular valve surface has a larger radius than that of said annular forward valve surface portion; a forward annular seal portion fitted in said forward annular groove portion and having a larger radius than that of the annular forward valve surface portion of said annular valve surface; and a rear annular seal portion contiguous with said forward annular seal portion and having an outer diameter that is substantially same as that of said forward annular seal portion but smaller than that of said annular rear valve surface portion, said rear annular seal portion being fitted and fixed in said rear annular groove portion.