1. A radiation therapy apparatus control method, comprising: calculating a plurality of degrees of similarity corresponding to a plurality of matching target areas based on a radiographic image in which a target subject is imaged; calculating a specific image position where a specific image area corresponding to a maximum value of said plurality of degrees of similarity among said plurality of matching target areas is positioned in said radiographic image; and calculating a drive amount with which a therapeutic radiation ray irradiation device that irradiates said target subject with a therapeutic radiation ray is driven, based on said specific image position, wherein an operation of calculating one degree of similarity corresponding to one matching target region out of said plurality of degrees of similarity includes: referring to a template table in which said plurality of matching target areas are associated with a plurality of sets of templates to calculate a set of templates corresponding to said one matching target area out of said plurality of sets of templates; and detecting, from said set of templates, a similar template most similar to a matching target image displayed in said one matching target area of said radiographic image, and wherein said one degree of similarity indicates a degree of similarity with which said matching target image is similar to said similar template.