PURPOSE: To impede the connection between acoustic modes in an above-floor compartment and an underfloor space so as to prevent the increase of noise in the above-floor compartment by disposing the blow-off port of circulated air in an aircraft at the ceiling center part in the above-floor compartment or at the ceiling center part in the underfloor space.

CONSTITUTION: A passenger compartment formed inside a body A in an aircraft is surrounded by interior material B and a floor C. An air blow-off port 1 is disposed at the ceiling center part in the passenger compartment, and an air inlet port 2 is disposed at the floor surface center part. The air in the passenger compartment is circulated through these ventilating openings 1, 2. The low-frequency region acoustic mode in the passenger compartment is made a node at the center part in the passenger compartment and an antinode at the wall surface in the passenger compartment. However, since ventilating openings 1, 2 are not disposed at the above-mentioned antinode part, the connection between the respective acoustic modes of the passenger compartment and a cargo compartment in the upper and lower parts of the floor C is impeded to prevent the increase of noise in the passenger compartment.