The present invention relates to a clamping apparatus of an injection molding machine, comprising: a fixed template; a moving template moving back and forth relative to the fixed template; a plurality of flanges provided with a template fastening hole extending in the horizontal direction so as to be fixed to the fixed template; and a plurality of booster cylinders fixedly mounted on the plurality of flanges and guiding the back and forth movement of the movable template, wherein the fixed template has a plurality of ribs, projecting outward, installed on one surface thereof, each of the plurality of ribs is provided with a seating groove that is depressed inward, and each of the plurality of flanges is disposed in the seating groove to adjust the coupling positions of the plurality of booster cylinders with respect to the fixed template.