Used in a method of this invention is a die clamping unit, which comprises a tie bar movably attached to a stationary platen, a halfnut positioning servomotor which advances and retreats to the tie bar, a halfnut which is provided on a movable platen and fixes the movable platen and the tie bar together by engaging the tie bar, an engaging mechanism which engages the halfnut with the tie bar, a hydraulic die clamping cylinder which presses the stationary platen and the movable platen, and a control device which controls the halfnut positioning servomotor and the hydraulic cylinder. The control device drives the halfnut positioning servomotor to remove a clearance between the halfnut and an engaging groove of the tie bar before a die clamping process carried out by the hydraulic cylinder.