1. A three-dimensional measuring method for executing three-dimensional measurement processing on a range where viewing fields of a plurality of cameras overlap by using images produced with the plurality of cameras, wherein the method comprising: a first step of accepting specification of a range of height to be measured; and a second step of extracting, with respect to an image produced with at least one of said cameras, an area where a point which is included in the viewing fields of all the cameras throughout the height range specified in the first step appears on the image by using a relation of a camera coordinate system between each of the cameras.