A mold clamping device comprising: a fixed platen which is installed securely on a machine base, has a mold clamping cylinder, and retains a fixed mold; a movable platen which is placed on the machine base, retains a movable mold, and is movable back and forth with respect to the fixed platen by a mold opening/closing cylinder; a plurality of tie bars, each of which is coupled at one end with a piston of the mold clamping cylinder, has at the other end part or the central part in the axial direction a screw or a ring groove formed in a serration shape or in a rectangular tooth shape, and penetrates through the movable platen; and mold clamping half nuts (206a,206b) which are attached to the movable platen and engageable with or disengageable from the screw or the ring groove of the tie bar; characterized in that the mold clamping device comprises a pair of additional half members (207a,207b) which are restricted in the axial direction of the tie bar by a nut box which is fixed to the movable platen and accommodates the mold clamping half nuts (206a,206b) and the additional half members (207a,207b), as with the mold clamping half nuts (206a,206b), and are moveable in an opening/closing direction of the half nuts (206a,206b) relative to the mold clamping half nuts (206a,206b), wherein, when the mold clamping half nuts (206a,206b) are closed and mated with the screw or the ring groove of the tie bar, the additional half members (207a,207b) are closed to hold the tie bar, and wherein the additional half members (207a,207b) have internally a serration-shaped internal screw or a serration-shaped ring groove which has the same shape as the external screw or the ring groove of the tie bar, and, when the additional half members (207a,207b) hold the tie bar by actuation of an actuator for opening and closing the additional half members (207a,207b), the serration-shaped internal screw or the serration-shaped ring groove is fitted into the external screw or the ring groove of the tie bar so as to remove play existing between the mold clamping half nuts (206a,206b) and the additional half members (207a,207b) by using wedge effects of inclined thread faces. a fixed platen which is installed securely on a machine base, has a mold clamping cylinder, and retains a fixed mold; a movable platen which is placed on the machine base, retains a movable mold, and is movable back and forth with respect to the fixed platen by a mold opening/closing cylinder; a plurality of tie bars, each of which is coupled at one end with a piston of the mold clamping cylinder, has at the other end part or the central part in the axial direction a screw or a ring groove formed in a serration shape or in a rectangular tooth shape, and penetrates through the movable platen; and mold clamping half nuts (206a,206b) which are attached to the movable platen and engageable with or disengageable from the screw or the ring groove of the tie bar; characterized in that the mold clamping device comprises a pair of additional half members (207a,207b) which are restricted in the axial direction of the tie bar by a nut box which is fixed to the movable platen and accommodates the mold clamping half nuts (206a,206b) and the additional half members (207a,207b), as with the mold clamping half nuts (206a,206b), and are moveable in an opening/closing direction of the half nuts (206a,206b) relative to the mold clamping half nuts (206a,206b), wherein, when the mold clamping half nuts (206a,206b) are closed and mated with the screw or the ring groove of the tie bar, the additional half members (207a,207b) are closed to hold the tie bar, and wherein the additional half members (207a,207b) have internally a serration-shaped internal screw or a serration-shaped ring groove which has the same shape as the external screw or the ring groove of the tie bar, and, when the additional half members (207a,207b) hold the tie bar by actuation of an actuator for opening and closing the additional half members (207a,207b), the serration-shaped internal screw or the serration-shaped ring groove is fitted into the external screw or the ring groove of the tie bar so as to remove play existing between the mold clamping half nuts (206a,206b) and the additional half members (207a,207b) by using wedge effects of inclined thread faces.