An injection molding machine is provided that includes a clamping force sensor that detects a clamping force of a mold, and a clamping force monitoring unit that monitors a detected value of the clamping force sensor and calculates a first mold open distance and/or a second mold open distance. The first mold open distance is calculated based on the difference between the maximum value of the detected values obtained during an injection process and a pressure holding process and the detected value obtained at the start of the injection process. The second mold open distance that is calculated based on the difference between the detected value obtained at the end of the pressure holding process and the detected value obtained at the start of the injection process.