PURPOSE: To reduce the number of parts, to simplify execution of a work, and to reduce a cost by a method wherein building is executed by using a building unit having size efficient to a width size regulation of a Road Traffic Act and the internal space of a building built by combination of building units is set to a single reference size to unify a module, in a unit building built in a state that a plurality of building units are arranged at intervals of a proper distance therebetween.

CONSTITUTION: A building unit 1 is formed such that the length of each side of the building unit 1 is set to lengths g×n+a and g×m+b obtained by adding other reference sizes (a) and (b) to multiples g×n and g×m of a single reference size (g). Installation distances (c) and (d) between the building units 1 are set such that values c+a and d+b obtained by adding the installation distances (c) and (d) and the other reference sizes (a) and (b) are a multiple of the single reference size (g).