Abstract: A tube coupler of this invention, is provided with a plug and a socket each having a fluid passage and a valve. When the plug and the socket are connected together, they push each other to cause the fluid passage of the plug to communicate with the fluid passage of the socket. The socket has a main socket tube body comprising an inner tube and an outer tube provided separately and threadably engaging each other. The plug has a main plug tube body insertable between the inner and outer tubes. The tube coupler has a seal member mounted on the outer peripheral surface of the inner tube of the main socket tube body for sealing a gap between the outer peripheral surface of the main socket tube body and the inner peripheral face of the main plug tube body.