To quickly move an axle load to a drive shaft side by shortening time intervals of output detection of a car height sensor for a prescribed time after adding the driving force to a two-axle car having a car height sensor which detects the car height at a prescribed frequency and controls based on the average value.

A driving force adding device is provided with computers 39, 40, sets the internal pressures of air springs 22, 24 in rear/front shaft 17 side and a rear/rear shaft 18 side according to the car load, detects them by pressure sensors 45, 46, controls a solenoid valve and a cut valve by the computer 9 so as to be set values, and feeds/exhausts air to/from the air spring 24 of the rear/rear shaft 18 side. A pair of car height regulating valves 29, 30 are controlled by the computer 39 so as to set the car height within a prescribed range at all times. In traveling, the frequencies of the pressure sensors 45, 46 and the car height sensor 41 are set to average values every 60sec., however, they are changed into the average values every 0.2 sec. for a prescribed time after changed into a state of driving force addition so that the moving time of the axle load can be shortened.