To inexpensively constitute a device for economizing air pressure consumed for adjusting the car height by an air spring for supporting a cab by a chassis frame.

Going in and out of the air pressure to the air spring at traveling time is prohibited by using an electric signal for indicating that a vehicle is in traveling, for example, an operating signal of a parking brake. This constitution can inexpensively constitute the device by using a solenoid valve and its control unit for prohibiting going in and out of the air pressure to the air spring when tilting the cab as the solenoid valve and its control unit for prohibiting going in and out of this air pressure.