To provide an underfloor humidity conditioning system capable of inhibiting increase of energy loss by reducing ventilation resistance in air circulation, and efficiently increasing humidity according to necessity.

A circulation humidity conditioning device 16 disposed in an underfloor space U, is provided with a plurality of humidity conditioning function boards 18, ... conditioning the humidity by contact with the air passing therethrough to condition the humidity of air introduced from an indoor space R through a suction-side ventilation hole 17 formed on a floor surface portion 4. Clearances 19 are respectively formed between the humidity conditioning function boards 18, ... at specific intervals to allow the air to pass therethrough. Five sheets of the humidity conditioning function boards are stacked in a state of directing to inner and outer directions in the vertical direction, and the humidity conditioning function boards 18, ... are arranged in parallel with each other in a state that the face extension direction is along the ventilating direction.