To provide a floor panel mounting structure in which a panel retainer is prevented from rotating in a dismounting direction insofar as a special rotary tool is not employed, by specially designing the shape of tool engaging depressions thereof.

According to the floor panel mounting structure, a panel bearer 13 is screwed onto a bearing leg 12 extending upward from a reference floor surface 1, and then locked at a desired height by a lock nut 14 screwed onto the bearing leg 12 from above the panel bearer 13. A corner portion of each floor panel 2 is borne on the panel bearer 13 and pressed from above by the panel retainer 20 screwed onto the bearing leg 12 or the lock nut 14. Further, a flange 21 of the panel retainer 20 has the plurality of tool engaging depressions 25 formed in an upper surface thereof at circumferentially regular intervals. Each tool engaging depression 25 is formed almost like a sphere, for instance. The special rotary tool 30 has a plurality of almost spherical projections 40 conforming in shape to the tool engaging depressions 25, at circumferentially regular intervals formed on a lower surface thereof. The number of each of the tool engaging depressions 25 and the projections 40 is preferably set to three.