Mold-clamping device, injection-molding device, and method for opening and closing mold

This mold-clamping device is provided with a fixed mold platen, a movable mold platen, and a mold-opening and -closing cylinder for moving the movable mold platen near to or away from the fixed mold platen, as well as a hydraulic pressure supply source for supplying hydraulic fluid to the mold-opening and -closing cylinder, and a control unit for driving and controlling the hydraulic supply source. The control unit is provided with: a flow rate reduction control unit during speed reduction for decreasing the flow rate of hydraulic fluid in accordance with a speed-reduction gradient set in advance when the movement speed of the movable mold platen is reduced in accordance with the speed-reduction gradient set in advance; and a flow rate increase control unit capable of performing control so as to increase the flow rate of the hydraulic fluid when the flow rate of the hydraulic fluid is being decreased in accordance with the speed-reduction gradient set in advance by the control unit.