In a floor panel structure of a vehicle body in which a floor of an automotive vehicle is formed by a floor panel coupled to a plurality of frame members extending in substantially longitudinal and width directions of the vehicle, there are provided a panel area formed by being enclosed at least by the frame members, a heavy-weight area located at a substantially central portion of the panel area, and a peripheral area located substantially around the heavy-weight area, wherein the heavy-weight area is configured so as to be heavier than the peripheral area. Accordingly, the vibration energy transmitted from the frame members to the floor panel can be reduced and thereby the acoustic emission generated from the floor panel can be reduced.