Abstract: During ordinary travel, a direction change cylinder (51) is actuated by an orbit roll (58) by turning a steering wheel (16) right and left, so that the rear wheels (4) can be turned for direction change in response to the operation of the steering wheel. When the direction change cylinder is actuated to the limit, the right and left rear wheels can be turned for direction change into an inclined state with their rear ends approaching each other, thereby making it possible to make a pivot turn. The front and rear wheel turning devices (30, 50) are actuated to turn the front wheels (3) and rear wheels around vertical axes (27, 49) for direction change by 90 degrees (directed exactly sideways). At this time, in the rear wheel turning device, with the direction change cylinder put in the neutral position, the rear wheel transverse travel cylinders (53) are actuated to turn the rear wheels for direction change by 90 degrees.