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 lateral acceleration acquisition unit configured to acquire a lateral acceleration 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 lateral acceleration based yaw rate computation unit configured to compute a lateral acceleration based yaw rate that is a yaw rate estimated based upon the lateral acceleration and the vehicle speed; a standard yaw rate computation unit configured to compute a standard yaw rate from the steering angle and the vehicle speed; and a pendular motion determination unit configured to determine whether or not a pendular motion caused by a swaying motion of the trailer is 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; and a determination unit configured to determine that the pendular motion caused by the swaying motion of the trailer is imparted, if the determination parameter is greater than the threshold value, characterized in that 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 a correlation of a value related to the lateral acceleration based yaw rate (Y'g) with a value related to the standard yaw rate (Y' s) and a correlation of a value related to the actual yaw rate (Y') with a value related to the standard yaw rate (Y' s). a yaw rate acquisition unit configured to acquire an actual yaw rate of the tractor; a lateral acceleration acquisition unit configured to acquire a lateral acceleration 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 lateral acceleration based yaw rate computation unit configured to compute a lateral acceleration based yaw rate that is a yaw rate estimated based upon the lateral acceleration and the vehicle speed; a standard yaw rate computation unit configured to compute a standard yaw rate from the steering angle and the vehicle speed; and a pendular motion determination unit configured to determine whether or not a pendular motion caused by a swaying motion of the trailer is 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; and a determination unit configured to determine that the pendular motion caused by the swaying motion of the trailer is imparted, if the determination parameter is greater than the threshold value, 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; and a determination unit configured to determine that the pendular motion caused by the swaying motion of the trailer is imparted, if the determination parameter is greater than the threshold value,