To enhance parts commonization between an axle housing of tread change type and an axle housing of a tread fixing type.

A front axle housing 20 is formed such that inner axles 20b, 20b are inserted slidably in right and left ends of an outer axle 20a and horizontal length can be extendable. Tread positioning devices 50, 50 are detachably mounted to both ends of the front axle housing 20 and tread fixing devices 60, 60 are detachably mounted to both ends of the front axle housing 20. In a work vehicle of tread fixing type, by removing the tread positioning devices 50, 50 and the tread fixing devices 60, 60, the outer axle 20a of tread changing type can be used in common.