A brake system includes a brake activator which controls, when a slipping condition is detected at a central rear wheel based on a signal detected by a central rear detector, the operating condition of a central rear brake based on the signal detected by the central rear detector so as to reduce a braking force on the central rear wheel lower than a braking force which is obtained as a result of operation of an input member. In this state, when an operation quantity of the input member increases as a result of operation of the input member, changing the operating conditions of both a right front brake and a left front brake, the brake activator increases the braking forces in both a right front wheel and a left front wheel according to an operation quantity of the input member.