To provide an underfloor storage structure which enables humidity conditioning and temperature conditioning of the entire underfloor space, and makes use of the entire space as a storage, and to provide a method of conditioning the humidity and temperature of the underfloor storage.

The underfloor storage is constructed such that an openable door is arranged in a floor plate of a house, for mounting goods on a storage bottom plate arranged under the floor plate and storing the goods in the storage, by opening/closing the openable door, and that air flow which is conditioned with respect to its temperature and humidity is circulated in the storage space between the floor plate and the bottom plate. According to the structure, circulating air is introduced into a double pipe which is embedded in a cobblestone sedimentary layer storing therein geothermy and in the ground, to thereby condition the temperature and humidity of the underfloor storage.