PURPOSE: To improve the operability by functioning a differential lock system when the steering rotary angle is smaller than predetermined level while releasing function when it is larger than predetermined level.

CONSTITUTION: If the handle is within predetermined range when the operating oil is pressure fed from a hydraulic pump 10 through the operating oil circulation system 9 in the arrow 27 direction and branched through a differential system 2A then fed to hydraulic motors 1a, 1b, the choking path 7 of differential lock system 5A is communicated through a controller 24 for receiving a steering angle detection signal to the differential system 2A. Said differential system 2A is employing a flow divider structure to feed equal amount of operating oil to both hydraulic motors 1a, 1b even if the loads on the left/right wheels are not equal thus to perform differential lock. Upon increase of steering angle, the communication path 6 of differential lock system 5A is communicated to the differ- ential system 2A to release differential lock.