Provided is a vehicle body behavior control device and a method of controlling behavior of a vehicle body which can reduce unstable behavior of the vehicle body. A vehicle body behavior control device incorporated into a vehicle body having a plurality of wheels includes: a brake mechanism which controls behavior of the wheels; and a control part which controls an interlocking brake operation in which a braking force is applied to the plurality of wheels using the brake mechanism when an operation for applying braking to any one of the wheels is performed based on a gradient value θ of a road surface on which the vehicle body travels.