To provide a mold clamping device which can clamp divided molds tightly.

A mold clamping device, which fastens molds in a state that the divided molds are joined and a workpiece is housed inside, includes fastening members 3 which are externally fitted to molds 2-1, 2-2 in a joined state and suppress mold opening by outward force in accordance with machining of the workpiece. A fitting surface of an outside of the molds and an inner side of the fastening member externally fitted to the molds is formed in a tapered surface inclining along a moving direction of the fastening member. A junction surface of the molds is tightly fastened by externally fitting the fastening member to the molds.