A pipe coupling is provided that facilitates an operation of opening or closing a ball valve and suppresses damage to a cylindrical seal member by the ball valve when rotated, thereby enabling an improvement in durability. A socket fluid passage (4) in a socket (1) is opened or closed with a ball valve (5). A cylindrical seal member (11) is disposed in a primary fluid passage portion (4a) of the socket fluid passage (4) in such a manner as to be in pressure contact with the ball valve (5) to seal between the ball valve (5) and the inner wall of the socket fluid passage (4). The ball valve (5) is rotatable to open when the socket (1) and a plug (2) are locked to each other by a lock mechanism (16). The ball valve (5) is provided with a sub-valve bore (33) that allows fluid in the primary fluid passage portion (4a) to be delivered to a secondary fluid passage portion (4b) of the socket fluid passage (4) through the ball valve (5) before a valve bore (7) of the ball valve (5) opens into the socket fluid passage (4) when the ball valve (5) is rotated.