PURPOSE: To improve performance of turning and steering by selecting a condition which a side brake can be operated or a condition which a reversing hydraulic clutch can be operated according to the condition of a working field, and freely changing the braking condition of the side brake at turning.

CONSTITUTION: A hydraulic operating type side brake 28 which is operated on one selected from a pain of right and left running devices, and a hydraulic clutch 30 with which reversing power can be transmitted to one selected from a pair of right and left running devices, are provided. A booster adjusting mechanism 36 is provided on a working oil feed system for in-operation of the side brake 28 and the hydraulic clutch 30, and connected with a manual operation lever 39. Further, a changeover means 49 capable of changeover to either of a first condition in which the side brake 28 can be operated but the hydraulic clutch 30 can not be operated, and a second condition in which the side brake 28 can not be operated but the hydraulic clutch 30 can be operated. The changeover means is cooperated with the booster adjusting mechanism 36 so that the boosting ratio of the hydraulic clutch 30 is larger than that of the side brake 28.