A socket to be attached to a liquid storing container having an opening formed in an upper surface thereof, the opening being formed with a cylindrical shape about an axial line, the socket comprising: a socket body including, formed therein, a socket liquid outflow passage that causes a liquid drawn out from a plug to be fixed to an inner peripheral surface of the opening to flow to an outside of the liquid storing container; a socket liquid inflow passage that guides the liquid flowing in from the outside of the liquid storing container to the plug; and a socket gas passage that circulates an outside air between an inside space of the liquid storing container and an outside space of the liquid storing container; and characterised by a ball lock mechanism including a plurality of lock balls; the ball lock mechanism allowing the plurality of lock balls to be engaged with engagement grooves and fixing the plurality of lock balls to the engagement grooves, respectively, the engagement grooves being formed along a circumferential direction about the axial line in an outer peripheral surface of the opening; a first cylindrical member having a plurality of opening holes, the opening holes being formed with a cylindrical shape about the first axial line and having a diameter smaller than an outer diameter of each of the lock balls; a second cylindrical member that is formed with a cylindrical shape about the first axial line and disposed at an outer peripheral side of the first cylindrical member, the second cylindrical member including a regulating portion that regulates the lock balls stored in the opening holes, respectively, to be engaged with the engagement grooves; and an urging force generation unit that urges the second cylindrical member against a location where the regulating portion contacts the lock balls, one end of the urging force generation unit being formed along the first axial line and fixed to the first cylindrical member, the other end of the urging force generation unit being formed along the first axial line and fixed to the second cylindrical member. a socket body including, formed therein, a socket liquid outflow passage that causes a liquid drawn out from a plug to be fixed to an inner peripheral surface of the opening to flow to an outside of the liquid storing container; a socket liquid inflow passage that guides the liquid flowing in from the outside of the liquid storing container to the plug; and a socket gas passage that circulates an outside air between an inside space of the liquid storing container and an outside space of the liquid storing container; and characterised by a ball lock mechanism including a plurality of lock balls; the ball lock mechanism allowing the plurality of lock balls to be engaged with engagement grooves and fixing the plurality of lock balls to the engagement grooves, respectively, the engagement grooves being formed along a circumferential direction about the axial line in an outer peripheral surface of the opening; a first cylindrical member having a plurality of opening holes, the opening holes being formed with a cylindrical shape about the first axial line and having a diameter smaller than an outer diameter of each of the lock balls; a second cylindrical member that is formed with a cylindrical shape about the first axial line and disposed at an outer peripheral side of the first cylindrical member, the second cylindrical member including a regulating portion that regulates the lock balls stored in the opening holes, respectively, to be engaged with the engagement grooves; and an urging force generation unit that urges the second cylindrical member against a location where the regulating portion contacts the lock balls, one end of the urging force generation unit being formed along the first axial line and fixed to the first cylindrical member, the other end of the urging force generation unit being formed along the first axial line and fixed to the second cylindrical member.