PURPOSE: To form a load bearing floor having sufficient strength simply and quickly.

CONSTITUTION: After a plurality of first floor boards 4 having notches interlocking with plumb members 3 are arranged to spread over the inside of a framework, second floor boards 5 having notches interlocking with the plumb members 3 are lapped so that they are not overlapped with joints of the first floor boards 4. A load bearing floor equipped with sufficient earthquake-resistance is constructed without causing such a trouble as slant slippage of the floor boards 4 or space between the floor boards by horizontal external force of an earthquake, etc., sufficient strength is also obtained, so that a finished floor body is formed easily by such simple work as double-layer floor boards are only mounted on framed cross members without causing any distortion or different sound by weight of walkers. According to the constitution, a construction period of time can be greatly reduced and, at the same time, construction can be executed at a low cost.