To provide an axle driving device with a HST and axles connected in association therewith in a housing, offering simple construction for allowing automatic shift to the side of a small deceleration ratio with the decreasing load of an engine and automatic shift to the side of a great deceleration ratio with the increasing load.

The axle driving device which has (a) a hydraulic pump 11 connected in association with the engine 2, (b) a hydraulic motor 21 driven while receiving a pressure oil from the hydraulic pump 11, (c) axles 50L, 50R connected in association with the hydraulic motor 21, stored in the housing, comprises displacement changing means 23 for changing the displacement of the hydraulic motor 21, load detecting means 201 for detecting the load of the engine 2 and control means 202 for controlling the displacement changing means 23 to change the discharge displacement of the hydraulic motor 21 in response to the load of the engine 2 detected by the load detecting means 201.