A brake system does not change immediately an operating condition of a left front brake although a slipping condition is detected at the left front wheel based on a signal detected by a left detector in such a state that a slipping condition is not detected at a right front wheel based on a signal detected by a right detector, and does not change immediately an operating condition of a right front brake although a slipping condition is detected at the right front wheel based on a signal detected by the right detector in such a state a slipping condition is not detected at the left front wheel based on a signal detected by the left detector.