1. A gear cutting simulation method comprising the steps of: creating, in a gear cutting simulation device, a gear-cutting-machine model which specifies relative positions between a blank model and a cutter model; creating a gear model as a result of simulation of gear cutting performed with the blank model and the cutter model arranged in the gear-cutting-machine model; outputting data indicating a tooth-contact state as a result of tooth-contact simulation performed by changing the relative positions between portions of the gear model which are arranged in an assembled state with one another; and comparing the outputted data indicating the tooth-contact state with a predetermined reference value, so that validity of the gear model is determined by the comparison result.