Provided is a control device for a radiation irradiation device including a position-related information acquisition portion that acquires position-related information related to a position of a particular site; a model generation portion that generates a model outputting the position of the particular site based on the position-related information; and a particular site position calculation portion that outputs the position of the particular site based on the position-related information acquired by the position-related information acquisition portion, by using the model generated by the model generation portion. The model generated by the model generation portion is provided with a filter regulating an amount of change in the position of the particular site output by the model.