To surely prevent a two-rear-axle vehicle in which the axle load distribution between the two rear axles is controlled according to live load from moving when stopped on a slope or the like even if a key switch is turned off.

When a two-rear-axle vehicle is vacant, for example, compressed air is fed from a tank 10 through a first magnet valve M1 opened and closed by a controller 20 to an air spring 8 working in the direction in which a driven rear rear axle is lifted, so as to hold the desired air pressure within the air spring 8 to make the axle load of a driving rear front axle greater than that of the driven rear rear axle; when a key switch is turned off, the controller 20 closes the first magnet valve M1 and a second magnet valve M2 for letting in outside air.