To make a wall thickness part in an axle box have an effect on weight without entailing any damage to outward appearance and also lubricating oil so as to be spread over a gear case without any hindrance.

In a traveling vehicle; installing a front-wheel axle case 2F and inserting an axle 4F, driving each front wheel 3F, into this axle case 2F, while attaching the front wheels 3F to both symmetrical projecting ends of the axle case 2F via a gear case 5F, a wall thickness part 6 being projected to the axle 4F toward this axle 4F side, is installed in an inner wall of the axle case 2F, and then an oil passage 7 leading to the gear case 5F is installed in this wall thickness part 6.