In order to calculate a ball screw abrasion amount of an injection-axis ball screw or a mold-clamping-axis ball screw, a position of a ball nut fitted to the ball screw is photographed by a camera installed in an injection molding machine. Then, if the photographed position of the ball nut is found, as a result of analysis of an image captured by the camera, to be deviated from the position of the ball nut when a ball screw has no abrasion, it is determined that the ball screw is abraded.