To provide a method of treating car-washing wastewater containing oil, mud, sand, a surface active agent and so on occurring in each car washing treatment place and so on by using a filter device using natural materials, which is environment-friendly, in which the treatment method is simple and the treatment effect is excellent.

The car-washing wastewater treatment method for comprise the following steps: (a) a slurry is formed by blending a filter aid with clear water; (b) the slurry is pre-coated on the filter surface of the first filter device; (c) the car-washing wastewater is filtrated by the first filter device, and the filtrated water obtained there is provided as washing water to car washing; (d) a part of the filtrated water of the first filter device is mixed with the filter aide to form slurry; and (e) by introducing a part of the filtrated water of the first filter device, or a part of the filtrated water and the pressurized air, the first filter device is backwashed to remove the cake. The device for the method is also provided.