A brake system (1701) does not change immediately the operating condition of a left front brake (1071) although a slipping condition is detected at the left front wheel (1031) based on a signal detected by a left detecting portion (1812) in such a state that a slipping condition is not detected at a right front wheel (1032) based on a signal detected by a right detecting portion (1822) and does not change immediately the operating condition of a right front brake (1072) although a slipping condition is detected at the right front wheel (1032) based on a signal detected by the right detecting portion (1822) in such a state a slipping condition is not detected at the left front wheel (1031) based on a signal detected by the left detecting portion (1812).
