Provided is a ball-lock connector wherein a connector which has a relatively small diameter and which can be smoothly attached or detached, is realized at low cost. Ball storage holes (23) are formed in a tubular member (20), and engagement balls (11) are contained in the ball storage holes (23). An inner sleeve (30) and an outer sleeve (40) are disposed on the outside of the tubular member (20) so as to move in the axial direction. When a counterpart connector (70) is attached, each engagement ball (11) moves from a first position wherein a part of the engagement ball is projected from the inner peripheral surface (21) of the tubular member (20) to a second position which is close to a first inner peripheral surface (42) of an outer sleeve (40), wherein a part of the engagement ball is not projected from the inner peripheral surface (21), and thereafter, returns to the first position. When the counterpart connector (70) is detached, each engagement ball (11) moves from the first position to a third position which is close to a second inner peripheral surface (44) of the outer sleeve (40), wherein a part of the engagement ball is not projected from the inner peripheral surface (21), and thereafter, returns to the first position.