1. A photomask manufacturing method for manufacturing a photomask group used to manufacture semiconductor devices and process a single layer, comprising: specifying a quality control location on a pattern formed after exposure by means of the photomask group; calculating quality control locations lying on photomasks contained in the photomask group and corresponding to the quality control location on the pattern and acquiring pattern position deviation information and pattern images of the quality control locations on the photomasks; simulating and generating a pattern image to be formed on a single layer of a semiconductor device by a plurality of exposures using a plurality of the photomasks contained in the photomask group, in a case where an exposure parameter at the time of exposure by means of the photomask group is changed, by using the acquired pattern position deviation information and the pattern images of the quality control locations on the photomasks contained in the photomask group, in the respective quality control locations for each exposure parameter; calculating a quality assurance value in the quality control location based on a plurality of thus formed pattern images; and calculating an exposure parameter with which an optimum quality assurance value is obtained based on a relationship between a setting value of the changed exposure parameter and a quality assurance value acquired from the pattern image formed by using the corresponding parameter.