An injection molding includes: a clamping mechanism (conversion mechanism and toggle mechanism) for moving a fixed platen that retains a fixed die toward a fixed platen that retains a fixed die to bring the fixed die and the movable die into contract, and generating a clamping force therebetween; a drive source (mold opening/closing motor) for driving the clamping mechanism; a clamping force detection unit for detecting the clamping force; an elongation value detection unit for detecting an elongation value indicating the amount of elongation of a tie bar that extends as the clamping force is generated; and an abnormality determination unit for determining an abnormality in the injection molding machine on the basis of the ratio of the elongation value and the clamping force.