The invention relates to a ring binder mechanism comprising a housing (10) for receiving two carrier rails (20), wherein the carrier rails are positioned adjacent one another with at least a portion of their inner longitudinal edges (23) facing each other to form a linkage axis (22) and at least a portion of their outer longitudinal edges (24) engaging the housing (10). The carrier rails (20) have at least two half-rings (16) rigidly connected with the carrier rails, which together form a ring (14). The carrier rails (20) are limitedly pivotable relative to each other about the linkage axis (22), taking along the half-rings (16), between an open position and a closed position. A locking rod (31) having at least one locking element (32) is movable parallel to the linkage axis (22) relative to the housing (10) and the carrier rails (20) via an actuating element (18). The locking rod is provided adjacent the bottom or exterior surface of the carrier rails (i.e. the surface facing away from the housing). The locking element (32) engages a portion of the carrier rails and blocks the pivot path of the carrier rails (20) in the closed position. Abutments or bearings are provided on the carrier rails (20) or on the housing flanks (28) for receiving locking forces, against which the locking element (32) is supported in the closed position.