A mold clamping device includes two mold platens; a ball screw mechanism connecting the mold platens to each other, the ball screw mechanism being provided with an axial force detector configured to detect an axial force acting on the ball screw mechanism; a servo motor provided on the ball screw mechanism and configured to drive the ball screw mechanism; and a control device configured to: monitor the axial force detected by the axial force detector during injection; and based on detecting an axial force increase, the axial force increase being an increase in the axial force, perform a mold clamping force increase control including driving the servo motor to increase a mold clamping force for tightening the ball screw mechanism.