To improve an execution property by suspending the side of a half- precast floor board not mounted on the beam of a building skeleton from the beam or column of the upper skeleton of the floor with slings.

A half-precast floor board 4 for an indoor floor board has a half size of a square section surrounded by four columns 1 of a skeleton in the square shape on the plan view. One long side section 4a of the floor board 4 and both short sides pinching the long side 4a are supported by beams 2 between the columns 1, and a long side 4c not mounted on the beam 2 is connected to the upper skeleton beam 3 of the floor by slings 6 in a suspended state at the time of execution. A half-precast floor board 5 for a balcony is likewise set to the suspended state, and concrete is placed on the floor boards 4, 5. No timbering is required to be provided on the lower floor of the execution floor, thus the lower floor can be executed in parallel, no beam or no cantilever beam is provided below the half-precast floor boards 4, 5, thus a neat direct ceiling can be formed, the floor height can be made low, and a ceiling horizontal duct can be easily installed.