PURPOSE: To allow load to a driving axle to be enlarged until the vehicle speed is increased when a wheel slips by continuing addition control for enlarging the allotment of load applied to a driving axle until the vehicle speed exceeds the second specified value when the time differential value of vehicle speed exceeds the specified value.

CONSTITUTION: The signals of a vehicle speed sensor 3 and a load sensor 2 for detecting the displacement of a leaf spring 1 are inputted to a control circuit 9, and a compressed air duct to an air spring for changing the allotment of load is controlled to open/close by driving a solenoid valve 8. Addition control for enlarging the allotment of load applied to a driving axle 5 is performed until the vehicle speed reaches the first specified value V1 from the extremely small value V0 when live load is the specified value L or less, and addition control is inhibited when the vehicle speed exceeds V1. Addition control is continued until the vehicle speed exceeds the second specified value V)2 larger than V1 after exceeding V1 when the time differential value of vehicle speed exceeds the specified value. The generation of slips of driving wheels on the road surface is thereby reduced so as to enable stable travel.