To provide a simplified floor structure with a catch basin.

The catch basin 5 is formed in a floor 10 of an area (room), in which equipment using oil is installed, in a reactor building. An inclined plane 4 inclined toward the catch basin 5 is formed on the floor 10. Ordinarily, drain piping 7 is tightly sealed with a cap 8. The catch basin 5 functions as an original catch basin when the drain piping 7 is tightly sealed with the cap 8. The oil leaked from the equipment using the oil flows along the inclined plane 4, and flows into the catch basin 5 tightly sealed with the cap 8. The oil is recovered into a vessel by a pump. The catch basin 5 functions as a drain collecting port when the cap 8 is detached from the drain piping 7. Leaked water leaked onto the floor 10 in maintenance inspection flows into the catch basin 5, and is discharged into the drain piping 7.