To provided a motion controller capable of adequately performing speed reduction control according to a degree of urgency representing the extent of the possibility that a vehicle becomes unstable.

A grip degree monitoring means M2 monitors a gripping degree representing the extent of lateral gripping of wheels and a start decision means M3 compares the gripping degree with a control start threshold to decide whether a speed reduction control means M1 starts performing speed reduction control of the vehicle according to the comparison result. Further, an urgency degree estimating means M4 estimates the degree of urgency representing the extent of the possibility that the vehicle becomes unstable based upon at least one of an operation state of the driver of the vehicle and the driving state of the vehicle (for example, the rate of variation of the gripping degree), and a threshold setting means M5 sets the control start threshold according to the degree of urgency.