A drive assist system includes an assist starting part starting assist, a detection part detecting relative distances and speeds between a vehicles, a calculation part calculating collision risks when changing a lane by the basis of the relative distances and speeds, a first judgment part judging whether the lane can be changed by the relative distances, speeds and the collision risks, a decision part deciding a target space for lane change by the relative distances and speeds when the lane cannot be changed, a second judgment part judging whether a lane changeable space is in the target space, a setting part setting a target speed for the vehicle go to a lane change waiting position when no space and to setting a target speed the vehicle enters a lane changeable position when there is the space, and a control part controlling a speed of the vehicle reaches the target speed.