To provide an installing structure of brake disk, capable of making compact and reducing weight of the unit.

An inner periphery setting part 18 of a brake disk rotor 11 is extended inwards of the radial direction, the inner periphery setting part 18 shares a wheel fixing flange, and a fixing means 24 for fixing the inner periphery setting part 8 is set at the vehicle outboard side end of a rotation part R so as to shorten the distance of an axis center C direction of the whole unit and enable reducing the weight.