The present invention provides a ring binder mechanism comprising a housing having a top portion and an open bottom generally opposed to the top portion; hinge plates supported by the housing for pivoting motion relative to the housing; rings for holding loose-leaf pages, each ring including a first ring member and a second ring member, the first ring member being mounted on a first hinge plate and moveable with the pivoting motion of the first hinge plate relative to the second ring member between a closed position and an open position, in the closed position the two ring members forming a substantially continuous, closed loop for allowing loose-leaf pages retained by the rings to be moved along the rings from one ring member to the other, and in the open position the two ring members forming a discontinuous, open loop for adding or removing loose-leaf pages from the rings; and a travel bar disposed between the top portion of the housing and the hinge plates and having a plurality of projections, at least one of the projections having a structure to block the hinge plates from pivoting upward relative to the housing and at least other one of the projections having a structure to allow the hinge plates to pivot upward relative to the housing. The ring binder mechanism according to the present invention may be opened by pulling only a pair of particular ring members and closed by pushing any pair of ring members, thereby reducing the risk of paper's falling out thanks to unintentional opening of the mechanism.