1. A scanning electron microscope device, comprising: a scanning electron microscope; a processor that composes an imaging recipe for imaging a specimen, which has patterns formed on the surface thereof, by the scanning electron microscope, and processes an image of the specimen obtained by imaging the specimen using the scanning electron microscope on the basis of the composed imaging recipe; a dimensional information extractor that extracts dimensional information on a pattern, which is formed on the specimen, from the image of the specimen processed by the processor; an input/output unit that inputs or outputs information to be processed by the processor and dimensional information extractor, information processed thereby, or design information on the specimen; and a control unit that controls the microscope, processor, dimensional information extractor, and input/output unit, wherein the processor includes: an index value calculation unit that calculates, on the basis of the design information inputted by the input/output unit, index values each indicating whether images of adjoining areas out of the plurality of images can be joined or are easy to join, a local imaging area group imaging recipe composition unit that composes an imaging recipe for a group of local imaging areas, which includes imaging areas of the respective images and an imaging magnification, on the basis of the index values which are calculated by the index value calculation unit and indicate easiness of joining, and a local image group joining unit that controls the scanning electron microscope on the basis of the imaging recipe composed by the local imaging area group imaging recipe composition unit, sequentially images a plurality of adjoining areas on the specimen while overlapping the adjoin areas, and joining a plurality of resultant images so as to produce a wide-area image, and wherein the dimensional information extractor extracts the dimensional information on the pattern from the wide-area image which the processor has produced by joining the plurality of images.