To sufficiently ventilate a building unit inside in a unit building.

A ventilating opening part 11 is formed in an external wall panel 3 of a unit building, a fire-resisting board 4 is provided inside the external wall panel 3 with some interval, and a ventilating passage 10 continuing to the opening part 11 is formed between the fire-resisting board 4 and the external wall panel 3. A ventilating opening part 15 is formed between the fire-resisting board 4 and a floor board 2. An intermediate board 6 is arranged between the fire-resisting board 4 and the external wall panel 3 with some interval from both of them, a ventilating passage 10 is formed between the intermediate board 6 and the fire-resisting board 4, an opening part 12 is formed in the intermediate board 6, and the opening part 12 of the intermediate board 6 and the opening parts 11, 12 of the external wall panel 3 are connected together by a duct 13. A vent cap 14 is attached to the opening part 11 in the external wall panel 3. The external wall panel 3 is made of ALC.