To provide a method and device for washing a car which is friendly to the environment, of a simple processing method, and has an excellent processing effect, in which wastewater of car wash containing oil, mud, sand, a surface active agent, and the like generated in each car wash station and the like is processed by a filtering device using a natural material.

In the method of car wash, at least a part of raw material water to be used for filtering is the car wash wastewater which is preliminarily filtered by a filter including a precoat type filter 15 and/or a bag filter 33. Precoat is applied to the filter surface of the preliminary filter 15, the car wash wastewater is filtered by the preliminary filter, the obtained water is supplied to a high level filter 24 as preliminary filtered water, and pure water obtained from the high level filter 24 is used for the car wash. A part of the filtered water from the filtering device of the precoat type preliminary filter 15 is mixed with a filtering aid to form a slurry, and the preliminary filter is backwashed to remove a cake film by introducing the part of the filtered water from the preliminary filter or the part of the filtered water and pressurized air.