The invention provides a runoff water purification system for removing pollutants which accumulate on road surfaces, thereby reducing the workload of the sewage water treatment plants and preventing environmental water pollution. The system 1 is installed under the ground of the side of the road 2. Runoff water runs into the system 1 through a curb inlet 120, is purified and discharged to a sewage pipe 56. The system is made up of a concrete box 11, in which a wall 10 separates the box into a maintenance tank 7 and a treatment tank 9, both of which are connected to each other at their lowest ends. Runoff water flows into the maintenance tank 7 and to the treatment tank 9, through the connecting conduit where pollutants are removed, and discharged to the sewage pipe.