A lock mechanism for a stage apparatus, includes a movable stage provided on a stationary member which is movable in a reference plane; a pair of engaging members which are provided on the movable stage; a pair of lock members which are positioned between the engaging members and are movable so as to move toward and away from corresponding the engaging members; a lock driving device which moves the lock members to an engaged position wherein the lock members mutually move away from each other and engage with the corresponding engaging members; and an unlock driving device which moves the lock members to a disengaged position wherein the lock members mutually move toward each other so as to disengage with the corresponding engaging members.