To prevent a lowering of productivity by surely preventing a a building from getting deficient of each unfit member with which a building unit is constituted.

When the building unit is assembled by weld joining each of pre-processed bar members and/or plate members (ceiling plates 1, flooring plates 2, front wall members 3, a rear wall member 4) using a building unit assembling device 5, either a specific assembling plan is chosen from many assembling plans information stored in a computer 59 or a specific assembling plan is input into the computer 59, and a characteristic value (length, width, thickness of the plate and others) of above each member is input the sensors 57, 58 before the weld joining procession, and input the value into the computer 59 so as to collate and confirm if the values fit with the specific assembling plan. The weld join processing is then followed. Each of the plate members 1, 2, 3, 4 is welded at its given point by a spot welding machine 55 at the weld join operation position (where a movable frame 53 is positioned).