Ring binder mechanism

The utility model relates to an annular holder device composed of a top, open-bottom housing opposite to the top, two hinging boards supported with the housing and in pivoting motion relative to the housing and a plurality of rings keeping the fillers. Each ring concludes two ring members. At least one of two ring members is arranged on the corresponding hinging board. The ring member can move between the closing position and the opening position along with the corresponding hinging board pivoting motion relative to the other ring member. A control structure is pivotally positioned. With the control structure, the hinging board is in pivoting motion; the ring member is at the closing state along with the pivoting motion of the hinging board. The width of the housing is smaller than that of width when two hinging boards are horizontally connected; the angle between the outer surfaces of two hinging boards is smaller than 180 degree. The annular holder device is simple in the structure, easy to be manufactured and low in the cost.