To properly operate a driving axle load increasing device from the beginning by releasing friction stick even if vibration such as traveling vibration from the outside is not applied.

In a driving axle load increasing device, a bypass pipeline 51 is put in connection between an air pressure source and an air spring 30, and a stick releasing valve 52 is connected to the bypass pipeline 51. High-pressure air is supplied to the air spring 30 through the stick releasing valve 52 in starting of the increase of driving axle load, and a rear-rear axle is highly lifted by the air spring 30.