This invention is to provide a technique capable of detecting a displacement of a positioning object with respect to a reference position in an actual space with high accuracy. A position of an imaging object detected in an imaged image is converted into a displacement of the positioning object with respect to the reference position in the actual space based on conversion information representing a correlative relationship between the position of the imaging object in the image and the displacement of the positioning object from the reference position. The conversion information is determined in advance based on positions of the imaging object detected from a plurality of images respectively imaged with the positioning object positioned at each of a plurality of imaging positions.