1. A centerline determining apparatus for determining a centerline of a body of a subject based on imaging data obtained from a part to be imaged including a torso part, a left-arm part, and a right-arm part of said subject, said apparatus comprising: an image producing unit for producing an image including a first region representing a cross section of said torso part, a second region representing a cross section of said left-arm part, and a third region representing a cross section of said right-arm part based on said imaging data; a binarizing unit for executing binarization processing for binarizing said image to obtain a binary image in which said first region, said second region, and said third region are represented by a first logic and a region outside of said part to be imaged is represented by a second logic; a separating unit for executing separation processing for separating said second region and said third region included in said binary image from said first region included in said binary image; an extracting unit for executing extraction processing for extracting said first region from an image obtained by said separating unit; and a unit for locating a centerline of the body of said subject by executing, in a case that said first region and one of said second region and said third region are included in an extracted region extracted by said extraction processing and the other of said second region and said third region is not included therein, first removal processing for removing said one region from said extracted region, and locating a centerline of the body of said subject based on said first-removal-processed extracted region.