A connector with a lock mechanism includes a connector 10 and a counterpart connector 20. A connector main body 11 of the connector 10 has an annular groove 113. An inner sleeve 17 and an outer sleeve are arranged movably in an axial direction on an outer circumferential surface of the connector main body 11 and outside the inner sleeve 17, respectively. The outer sleeve 13 has a locking pin 15 projecting from an inner circumferential surface close to a front end of the outer sleeve 13. A ball accommodation hole 22, an inlet groove 26, and a circumferential direction groove 28 are formed in a tubular member 21 of the counterpart connector 20. When the connector main body 11 is inserted into the tubular member 21, an engaging ball 23 accommodated in the ball accommodation hole 22 becomes engaged with the annular groove 113 and the locking pin 15 becomes engaged with the inlet groove 26, allowing the connectors to be connected with each other. When the outer sleeve 13 is rotated with respect to the tubular member 21, the locking pin 15 becomes engaged with the circumferential direction groove 28 and restricts the movement of the outer sleeve 13 and the connector main body 11 in the axial direction.