To facilitate an escape when a driving wheel of a vehicle having a trunnion type suspension falls in a recessed part of a road surface or the like.

A cylinder mechanism 50 is connected to a link 45 arranged in a center clamp part 22 of superposed leaf springs 20. An air pressure circuit 70 is connected to the cylinder mechanism 50. A displacement detecting sensor 60 is arranged to output a signal according to a forward inclination of the superposed leaf springs 20. The air pressure circuit 70 supplies compressed air to the cylinder mechanism 50. When a driving wheel falls in a recessed part, the sensor 60 sends out a signal according to a degree of a forward inclination of superposed leaf springs 20, and comes to supply pressure air being higher as a forward inclination is large to the cylinder mechanism 50. The front end side of the superposed leaf springs 20 is pressurized downward by this cylinder mechanism 50, and since grounding pressure of the driving wheel increases, the eascape from the recessed part is facilitated.