A mold clamping apparatus for injection molding machines includes a stationary plate 2 for supporting a stationary mold Cc; a moving plate 6 for supporting a moving mold Cm; a plurality of parallel tie bars 3a, 3b, 3c, and 3d of which end portions are secured to the stationary plate 2; a plurality of different cylinders 4a-4d . . . which accommodate piston sections 5a, 5b, 5c, and 5d set at the middle of each tie bar, and arranged movably along the tie bars and integrally with the movable plate; and a hydraulic circuit 7 capable of selective feeding of pressurized oil to the oil chambers of the cylinders. The piston sections of different cylinders are formed in different diameters and all portions of all tie bars are formed in the same diameter.