To efficiently use an under-floor space, which is yet an above-floor space, as a storage space while the storage space is ventilated using the air under the floor.

A floor level of the storage space 3 is lowered to the earth floor level of an under-floor space 9, and also provided is an air vent 11 communicating the storage space 3 and the adjacent under-floor space 9 on the earth floor level. Then, an air vent 7 is provided in a partition 6 between the storage space 3 and an adjacent space 1. Also, an under-floor inspection port 11 is provided in a partition 10 between the storage space 3 and the adjacent under-floor space 9. Furthermore, a ventilation device 12 is provided in the partition 10 between the storage space 3 and the adjacent under-floor space 9. Inside the storage space 3, a floor material 8 leading to a floor surface 4 of the adjacent space 1 is provided.