To improve the reliability and assembling workability of an axle device by cooling a lubricating oil inside a casing with a simple structure.

A heat-transfer plate 41 is provided inside the casing 13. This heat-transfer plate 41 is formed in such a way that a mounting plate 41A of the basal side thereof is mounted on a cover 15 of the casing 13 and an oil contacting portion 41D of the apical end side thereof is extended downwards to a position where it is immersed in the lubricating oil 38. Accordingly, the heat-transfer plate 41 can positively transfer the heat generated in heating the lubricating oil 38 to the side of the cover 15 by directly immersing the oil contacting portion 41D into the lubricating oil 38, so that this heat can be diffused from a heat dissipating fin member 43 into the atmosphere. As a result, the lubricating oil 38 inside the casing 13 can be effectively cooled by providing only one heat-transfer plate 41.