To ventilate an outdoor floor slab without chipping a waterproof layer of the outdoor floor slab.

This ventilation structure is composed of a floor framework 12 for forming an underfloor space 11B of an outdoor floor Y arranged outdoors, and an outdoor floor backing material 13 of the outdoor floor Y for forming a ventilation passage 11A in a lower part; and has the outdoor floor slab 10 for arranging the outdoor floor backing material 13 on an upper surface of the floor framework 12, a handrail wall (a peripheral wall) 20A and a habitable room wall (a peripheral wall) 20B having a ventilation space 21 on a side surface on the outdoor floor Y side; and is characterized in that a lower story living room SK is positioned under the underfloor space 11B, and the ventilation passage 11A of the outdoor floor slab 10 communicates with the ventilation space 21 of the handrail wall 20A and the habitable wall 20B.