To keep a floor level.

An internal skeleton part having the floor (1) and an external skeleton part (5) serving as a ground sill are made independent of each other, the internal skeleton part is made round, and a weight (4) is placed in the internal skeleton part, which is fitted into the skeleton part (5) hollowed out in the same round shape as the internal skeleton part in such a state that the section between both of the skeleton parts is made slippery. Consequently, the floor (1) is kept level because the section having the weight (4) of the internal skeleton part is constantly located in a lower position.