When a brake of a tractor is released, a control unit having a computer built therein monitors the pressure of working fluid output from a trailer control valve that controls a brake of the trailer in conjunction with the brake of the tractor. When time required for a pressure of the working fluid to decrease to a predetermined pressure is a predetermined time or longer, the control unit determines that the trailer is coupled with the tractor. When the time is less than the predetermined time, the control unit determines that the trailer is not coupled with the tractor.