Provided is a vehicle behavior controller capable of stabilizing a behavior of a vehicle with a high response when an unstable state of the vehicle is judged. The vehicle behavior controller includes: a running state detecting unit for detecting a running state of the vehicle; an unstable state judging unit for judging the unstable state of the vehicle based on the running state; a motor generator connected to an engine of the vehicle, the motor generator operating as a power generator for using an output of a prime mover to generate power and an electric motor for assisting the output of the prime mover; and a control unit for controlling an operation of the motor generator based on r.p.m. of the motor generator, in which the control unit controls the operation of the motor generator to generate braking torque when the unstable state is judged.