A driving support system detects a deceleration object ahead and starts deceleration assistance for a host vehicle. The driving support system includes a reliability calculation unit configured to calculate a reliability of detection of the deceleration object, an assistance mode selection unit configured to select an assistance mode of the deceleration assistance from among multiple assistance modes in accordance with the calculated reliability of detection, and a driving support unit configured to perform the deceleration assistance in the selected assistance mode. The assistance mode selection unit is configured to select a first assistance mode when the reliability of detection is lower than a predetermined reliability threshold and select a second assistance mode higher in a level of the deceleration assistance than the first assistance mode when the reliability of detection is higher than or equal to the reliability threshold.