To continuously detect a roll angle, in a roll angle detector of a motorcycle.

An ultrasonic transmitter 19 is arranged facedly to a road surface Ro in the center of a vehicle body, and ultrasonic receivers are arranged right and left. Time data is coded and AM-modulated, an ultrasonic signal is transmitted from the ultrasonic transmitter 19, and the ultrasonic signal reflected on the road surface Ro is received by the right and left ultrasonic receivers 20 and 21. Based on the time data included in the ultrasonic signal, the transmission time of the ultrasonic signal from the ultrasonic transmitter 19 to the right and left ultrasonic receivers 20 and 21 is determined, and the roll angle of the vehicle body is calculated based on the transmission distance of ultrasonic signal. Using the time data allows the continuous detection of the roll angle.