A rider-assistance system of a straddle-type vehicle comprising: a surrounding environment detector that detects surrounding environment behind the straddle-type vehicle; a processor; and an execution device that executes assistance operation, wherein the processor includes: an acquisition section that acquires surrounding environment information about the straddle-type vehicle; a determination section that determines necessity of assistance operation executed by the rider-assistance system to assist with a rider's operation on the basis of the surrounding environment information acquired by the acquisition section; and a control section that outputs a control command to the execution device to execute the assistance operation in the case where the determination section determines that the assistance operation is necessary, the surrounding environment information includes information on surrounding environment behind the straddle-type vehicle, the determination section determines the necessity of the assistance operation by using a rear index value that is an index value depending on the information on the surrounding environment at the rear and is an index value of a collision possibility of an object located behind the straddle-type vehicle against the straddle-type vehicle, the rear index value is a value that varies according to a relative distance (Dr) and a relative speed (Vr) of the object located behind to the straddle-type vehicle, the surrounding environment information is information corresponding to output of the surrounding environment detector, the execution device includes a speed controller that is provided to the straddle-type vehicle, and the assistance operation includes speed correction operation that is executed by the speed controller to increase a target acceleration or a target speed generated by an adaptive cruise control of the straddle-type vehicle. a surrounding environment detector that detects surrounding environment behind the straddle-type vehicle; a processor; and an execution device that executes assistance operation, wherein the processor includes: an acquisition section that acquires surrounding environment information about the straddle-type vehicle; a determination section that determines necessity of assistance operation executed by the rider-assistance system to assist with a rider's operation on the basis of the surrounding environment information acquired by the acquisition section; and a control section that outputs a control command to the execution device to execute the assistance operation in the case where the determination section determines that the assistance operation is necessary, an acquisition section that acquires surrounding environment information about the straddle-type vehicle; a determination section that determines necessity of assistance operation executed by the rider-assistance system to assist with a rider's operation on the basis of the surrounding environment information acquired by the acquisition section; and a control section that outputs a control command to the execution device to execute the assistance operation in the case where the determination section determines that the assistance operation is necessary, the surrounding environment information includes information on surrounding environment behind the straddle-type vehicle, the determination section determines the necessity of the assistance operation by using a rear index value that is an index value depending on the information on the surrounding environment at the rear and is an index value of a collision possibility of an object located behind the straddle-type vehicle against the straddle-type vehicle, the rear index value is a value that varies according to a relative distance (Dr) and a relative speed (Vr) of the object located behind to the straddle-type vehicle, the surrounding environment information is information corresponding to output of the surrounding environment detector, the execution device includes a speed controller that is provided to the straddle-type vehicle, and the assistance operation includes speed correction operation that is executed by the speed controller to increase a target acceleration or a target speed generated by an adaptive cruise control of the straddle-type vehicle.