To provide a floating supporting structure capable of certainly attaching an attached member to an attaching member (or detaching it from the attaching member).

In a collar member 21, a large-diameter flange section and a cylindrical section 21a are integrally formed, and a female screw 21d is formed on the inner peripheral surface of the cylindrical section 21a. A stopper section 21e is disposed in the large-diameter flange section of the collar member 21. In a vibration proofing member 20, the cylindrical section 21a is inserted into a through hole 20d of the tube section 20a, the collar member 21 is engaged with it, and the stopper section 21e is mounted to a pivot holder 16 so that it is located between both opening ends 17a (or 18a) of an attaching section 17 (or 18) of the pivot holder 16. The pivot holder 16 is floating-supported by a car body by screwing a bolt 32 with a female screw 21d of the collar member 21.