The present invention provides a building construction method and a room module which can realize reduction in a construction period in addition to reduction in an on-site working amount by further enlarging a room modularization range. The present invention uses four column members which are vertically provided at inner corners of four room modules adjacent to one another as a frame column of the building when a plurality of room modules are installed in a grid pattern on a floor surface of the building. Specifically, the present invention uses the column member of an angular steel pipe, which the room module has, as the frame column of the building. Thereby, on a construction site, it is not necessary to construct the frame column in advance, and the frame column can be constructed by using the column members of the room module while the room module is installed on the site.