PURPOSE: To do away with stepped difference, by forming an outside floor framework composed of outerside frame members and an inside floor framework assembled by positioning it lower by a specified dimension from the upper face of the outside floor framework and provided with tatami mats via plate members.

CONSTITUTION: The outside floor framework of floor panels 6 of a Japanese style room unit A is constituted of end joints 8a, 8c and side joist 8b at the outside and the inside floor framework is constituted of end joists 9a, 9c and side joist 9b at the inside, which are made lower than a specified depth (h) in the height of the sideface. This specified depth (h) is designed to be the figure that the thickness of floor face-finished members 20 placed on the upper face of floor panels 13 of a Western style unit B1 adjacently arranged in subtracted from the thickness of tatami mats 21 placed on the floor panels (b). Next, the units A1, B1 are adjacently arranged to each other and an entrance and an exit are provided at the wall panel 7a of the unit A1. The floor panel 13 is formed higher by a specified level (h) than the floor panel 6. In this way, the stepped difference can be gotten rid of by use of thicker tatami mats by (h) than the finished floor 20.