PURPOSE: To make it possible to enhance the structural strength of the whole unit building without processing a reference plane and reduce on-site work and connect building units with ease.

CONSTITUTION: Joint structure 1 for a main structural material for a unit building is designed to interconnect posts of adjacent building units 12 and 13, which are separated from each other with a clearance (C). A first nut 2 installed to a post 15A for a building unit 13 on one side and a second nut 3 installed to a post 15B of a building unit 13B or the like on the other side, screws 4 which can be threaded into these nuts 2 and 3 and a regulating member 5 which regulates the span between the posts 15A and 15B are provided. This construction eliminates the need to process the reference plane for the posts 15 or the like, which makes it possible to make an accurate alignment for equipment.