System for detecting stability/instability of behavior of a motor vehicle upon occurrence of tire slip or lock. State of the motor vehicle is determined on the basis of an alignment torque (Ta) applied from a road and a side slip angle (.beta.). By taking advantage of such torque/slip-angle characteristic that although the alignment torque is proportional to a side slip angle when the latter is small, the alignment torque becomes smaller as the side slip angle increases, a normal value is determined from a straight line slope and the side slip angle in a region where the latter is small. Unstable behavior of the motor vehicle is determined when deviation of actual measured value from the normal value increases. Further, unstable state is determined when the slope of the alignment torque for the slip angle departs significantly from that of approximate straight line slope.