An assembling method that is high in assembly accuracy and easily performed at the actual installation site is provided. In this method, the sub base plates on the sub mounting tables are aligned with respect to a main base plate on a main mounting table at a temporary installation site, and the state thereof is maintained. Then, the main mounting table and the sub mounting tables are separated from each other to be transferred to the actual installation site, and the positioning state is restored at the actual installation site. Since a positional alignment can be performed at the temporary installation site, the work required at the actual installation site can be reduced. Furthermore, since a fine adjustment can be performed at the actual installation site, the accuracy in the positional alignment can be improved.