In the floor base panel (40), an area on the support board (33) of the vibration proof support leg (30) is formed solid, and in the other area, a plurality of cavities (41) extending in the parallel direction to the floor surface are formed. Thereby, because the vibration energy propagated from the impact point of the floor base panel (40) is reflected on the boundary between the cavity formation area and the sold area, and repeatedly passes the cavity formation area, it is attenuated in a short time, and the vibration energy or sound energy transmitted to a slab (20) can be reduced.