A driver assistance system 1 for a motorcycle is provided with: a driving force output device 82 that generates a backup output for reversing a drive wheel and backing up a vehicle body; a brake lever 810 squeezably operable by a rider; a ACC main switch 43 and an ACC lever 44 operable by the rider; and a back out assistance controller 64 that controls the driving force output device 82 on the basis of the operation of the lever 810, the switch 43, and the lever 44. The back out assistance controller 64, after causing the driving force output device 82 to generate the backup output when triggered by receiving an operation of turning the lever 44 to a “SET/−” side, continues to cause the backup output to be generated until the lever 810 is operated, or until the switch 43 or lever 44 is turned off.