The present invention realizes a common use of left and right gear casings aims thus reducing a manufacturing cost by an amount corresponding to the common use of the left and right gear casings. According to the present invention, in a tractor in which a front axle case which extends in the lateral direction is mounted on a body frame, and a pair of left and right front wheels are respectively mounted on left and right end portions of the front axle case by way of left and right gear casings, the front axle case is constituted of a center casing forming body and left and right casing forming bodies which have respective proximal end portions thereof connected to left and right end portions of the center casing forming body, and proximal end portions of the left and right casing forming bodies have mounting positions thereof arranged adjustable about axes with respect to the left and right end portions of the center casing forming body.