The present invention is a floor panel (2) structure of a car body where an automobile floor includes a floor panel that is connected to the frame members (12,14,20,22) of the car body and also have a vibration mode adjusting structure that suppresses the generation of acoustic emission by generating 2×1 mode vibrations in a predetermined frequency band. The floor panel has a non-rectangular vibration region and the vibration mode adjusting structure of the floor panel has a vibration adjusting part that defines the position of node of 2×1 mode vibration, and this vibration adjusting part is disposed within the non-rectangular vibration region so as not to touch the frame members and so as to form two regions where the vibration volumes of two antinodes of vibration in 2×1 mode are made nearly the same.