A motion stabilizer for a tractor to which a trailer is connectible, the motion stabilizer comprising: a yaw rate acquisition unit configured to acquire an actual yaw rate of the tractor; a steering angle acquisition unit configured to acquire a steering angle of the tractor; a vehicle speed acquisition unit configured to acquire a vehicle speed of the tractor; a standard yaw rate computation unit configured to compute a standard yaw rate (Ys) from the steering angle and the vehicle speed; characterized by further comprising a pendular motion determination unit configured to determine whether or not a pendular motion of the tractor is caused by a swaying motion of the trailer being imparted to the tractor, the pendular motion determination unit including: a parameter computation unit configured to compute a determination parameter indicative of a quantity of change in the actual yaw rate; a threshold value setting unit configured to compute a threshold value (PAth) for the determination parameter; a determination unit configured to determine that the pendular motion of the tractor is caused by the swaying motion of the trailer being imparted to the tractor, if the determination parameter is greater than the threshold value; and an evaluation value computation unit configured to compute a steering-back maneuver evaluation value indicative of a possibility of making periodical steering-back maneuvers, based upon at least one of a value related to the actual yaw rate and a value related to the standard yaw rate, wherein the pendular motion determination unit is configured to make a change to at least one of the threshold value and the determination parameter based upon the steering-back maneuver evaluation value. a yaw rate acquisition unit configured to acquire an actual yaw rate of the tractor; a steering angle acquisition unit configured to acquire a steering angle of the tractor; a vehicle speed acquisition unit configured to acquire a vehicle speed of the tractor; a standard yaw rate computation unit configured to compute a standard yaw rate (Ys) from the steering angle and the vehicle speed; characterized by further comprising a pendular motion determination unit configured to determine whether or not a pendular motion of the tractor is caused by a swaying motion of the trailer being imparted to the tractor, the pendular motion determination unit including: a parameter computation unit configured to compute a determination parameter indicative of a quantity of change in the actual yaw rate; a threshold value setting unit configured to compute a threshold value (PAth) for the determination parameter; a determination unit configured to determine that the pendular motion of the tractor is caused by the swaying motion of the trailer being imparted to the tractor, if the determination parameter is greater than the threshold value; and an evaluation value computation unit configured to compute a steering-back maneuver evaluation value indicative of a possibility of making periodical steering-back maneuvers, based upon at least one of a value related to the actual yaw rate and a value related to the standard yaw rate, a parameter computation unit configured to compute a determination parameter indicative of a quantity of change in the actual yaw rate; a threshold value setting unit configured to compute a threshold value (PAth) for the determination parameter; a determination unit configured to determine that the pendular motion of the tractor is caused by the swaying motion of the trailer being imparted to the tractor, if the determination parameter is greater than the threshold value; and an evaluation value computation unit configured to compute a steering-back maneuver evaluation value indicative of a possibility of making periodical steering-back maneuvers, based upon at least one of a value related to the actual yaw rate and a value related to the standard yaw rate, wherein the pendular motion determination unit is configured to make a change to at least one of the threshold value and the determination parameter based upon the steering-back maneuver evaluation value.