PURPOSE: To enable a construction machine having freely turnable front and rear wheels to easily make a small turn at a corner and narrow the width toward a neighouring object by simplifying the operation of a front steer, a counter steer, a crab steer and the like.

CONSTITUTION: Spools for direction control valves 17, 18 are connected to a steering handle shaft 16. Solenoids 33 for electromagnetic valves 22 to 26 are made freely switchable by a mode changeover switch and each of electromagenetic valves 22 to 26 is changed over based on the steering mode set by the mode changeover switch. The directions of front wheels 1, 1 and 2, 2 and rear wheels 3, 3 and 4, 4 as well are changed under an established steering mode by making front and rear wheel hydraulic cylinders 9a, 9b, 11a, 11b, 13a, 13b, 15a, 15b contract and expand by interlocking with the steering operation of a steering handle 5. Furthermore, only the hydraulic cylinder for the drag arm of the rear wheel is made capable to contract and expand by providing a rear independent steer switch.