A control system which controls behavior of a motorcycle, the system comprising: an obstacle position information acquiring unit that acquires obstacle position information serving as position information of an obstacle existing ahead of the motorcycle; a determination unit that determines whether or not the obstacle is located in a predetermined range where a collision avoiding operation of the motorcycle is required, based on the obstacle position information acquired by the obstacle position information acquiring unit; an execution unit that causes the motorcycle to execute an automatic decelerating operation in a case where it is determined by the determination unit that the obstacle is located in the predetermined range; a lane position information acquiring unit that acquires lane position information serving as relative position information of lane boundaries (LV_R, LV_L) with respect to the motorcycle during traveling; and a detection angle range setting unit that sets a detection angle range (Rθ) of a forward environment detecting device for acquiring the obstacle position information, wherein the forward environment detecting device comprises a first sensing system and a second sensing system, wherein a detection angle range (Rθ) of the first sensing system is smaller than a detection angle range (Rθ) of the second sensing system, characterised in that the detection angle range setting unit determines to use the second sensing system of the forward environment detecting device in a case where a determination reference is satisfied, wherein the determination reference includes a condition that the lane position information acquired by the lane position information acquiring unit satisfies a prescribed condition, and wherein the prescribed condition is a condition that the lane position information acquired by the lane position information acquiring unit is information indicating a state where a distance from the lane boundary (LV_L) closest to the motorcycle to the motorcycle is smaller than a reference value temporarily or over a period longer than a reference period. an obstacle position information acquiring unit that acquires obstacle position information serving as position information of an obstacle existing ahead of the motorcycle; a determination unit that determines whether or not the obstacle is located in a predetermined range where a collision avoiding operation of the motorcycle is required, based on the obstacle position information acquired by the obstacle position information acquiring unit; an execution unit that causes the motorcycle to execute an automatic decelerating operation in a case where it is determined by the determination unit that the obstacle is located in the predetermined range; a lane position information acquiring unit that acquires lane position information serving as relative position information of lane boundaries (LV_R, LV_L) with respect to the motorcycle during traveling; and a detection angle range setting unit that sets a detection angle range (Rθ) of a forward environment detecting device for acquiring the obstacle position information, wherein the forward environment detecting device comprises a first sensing system and a second sensing system, wherein a detection angle range (Rθ) of the first sensing system is smaller than a detection angle range (Rθ) of the second sensing system, characterised in that the detection angle range setting unit determines to use the second sensing system of the forward environment detecting device in a case where a determination reference is satisfied, wherein the determination reference includes a condition that the lane position information acquired by the lane position information acquiring unit satisfies a prescribed condition, and wherein the prescribed condition is a condition that the lane position information acquired by the lane position information acquiring unit is information indicating a state where a distance from the lane boundary (LV_L) closest to the motorcycle to the motorcycle is smaller than a reference value temporarily or over a period longer than a reference period.