A tube connector comprising: a plug having a plug channel through which a fluid flows and a ball engagement groove with which a ball engages, the ball engagement groove being formed along an outer periphery of the plug; and a socket to be fitted around a terminal end of the plug and having a socket channel that forms a channel for the fluid together with the plug channel, wherein the socket includes a tubular socket body and a sleeve, the socket body having an insertion hole into which the terminal end of the plug is inserted and a ball-receiving hole capable of receiving the ball, the sleeve being provided on an outer periphery of the socket body and reciprocable relative to the socket body in an axial direction between a base-end position and a terminal-end position, the sleeve being provided with a projection that projects radially inward, wherein when the plug and the socket are fitted to each other while the sleeve is set in the terminal-end position, the projection causes the ball disposed within the ball-receiving hole to engage with the ball engagement groove of the plug so as to regulate relative movement between the plug and the socket in the axial direction and secure the plug and the socket to each other, characterized in that when the plug is inserted into the socket while the sleeve is set in the terminal-end position, insertion of the plug is permitted by fitting the ball into a gap between a ring and the projection, a biasing force of a spring being applied on the ring, and an accommodation space in the sleeve radial outside of the ball receiving holes of the socket body is provided such that, when the plug is inserted into the insertion hole of the socket while the sleeve is set in the base-end position, if the ball rolls on the outer periphery of the plug so as to come into abutment with a wall that forms the ball-receiving hole and/or with the projection, the accommodation space entirely accommodates the ball inside the sleeve and the socket body relative to the outer periphery of the plug. a plug having a plug channel through which a fluid flows and a ball engagement groove with which a ball engages, the ball engagement groove being formed along an outer periphery of the plug; and a socket to be fitted around a terminal end of the plug and having a socket channel that forms a channel for the fluid together with the plug channel, wherein the socket includes a tubular socket body and a sleeve, the socket body having an insertion hole into which the terminal end of the plug is inserted and a ball-receiving hole capable of receiving the ball, the sleeve being provided on an outer periphery of the socket body and reciprocable relative to the socket body in an axial direction between a base-end position and a terminal-end position, the sleeve being provided with a projection that projects radially inward, wherein when the plug and the socket are fitted to each other while the sleeve is set in the terminal-end position, the projection causes the ball disposed within the ball-receiving hole to engage with the ball engagement groove of the plug so as to regulate relative movement between the plug and the socket in the axial direction and secure the plug and the socket to each other, characterized in that when the plug is inserted into the socket while the sleeve is set in the terminal-end position, insertion of the plug is permitted by fitting the ball into a gap between a ring and the projection, a biasing force of a spring being applied on the ring, and an accommodation space in the sleeve radial outside of the ball receiving holes of the socket body is provided such that, when the plug is inserted into the insertion hole of the socket while the sleeve is set in the base-end position, if the ball rolls on the outer periphery of the plug so as to come into abutment with a wall that forms the ball-receiving hole and/or with the projection, the accommodation space entirely accommodates the ball inside the sleeve and the socket body relative to the outer periphery of the plug.