Provided is a driving assist system which includes an own vehicle velocity sensor, an adjacent vehicle velocity sensor, and a control device and where the control device makes constant velocity travelling by causing a velocity of an own vehicle to fall within a constant velocity range between a lower limit value and an upper limit value set based on a target velocity and, when the own vehicle travels on a passing lane and an adjacent vehicle travels on a cruising lane, if a relative velocity of the adjacent vehicle to the own vehicle falls within a set range, the control device performs speed-up adjustment to increase the velocity of the own vehicle in a range up to the upper limit value.