Front wheels (6L, 6R) a steering operation of which is possible by a steering mechanism (12) are mounted on a vehicle body (2). Steering cylinders (22L, 22R) of the steering mechanism (12) extend and contract in response to an operation of a steering handle (27) provided in a cab (5) to perform the steering operation of the front wheels (6L, 6R). The steering mechanism (12) is provided with a rotation angle sensor (28) for detecting a steering angle θ of the left front wheel (6L) and a proximity switch (29) for detecting whether or not the front wheel (6L) is in a straight-ahead state. A controller (30) determines that the rotation angle sensor (28) or the proximity switch (29) is abnormal when the steering angle (θ) detected by the rotation angle sensor (28) is different from the detection result by the proximity switch (29).