To obtain an apparatus for treating waste water when a car is washed, which has stable filtration performance, by which the filtrate so purified at high level can be obtained that almost all of wax, various kinds of oil, surfactants, suspended materials and the like are removed and bacteria or the like does not propagate themselves, whose maintenance is not necessary for a long period of time and from which secondary industrial waste or the like is not generated.

This apparatus is constituted by arranging successively a pre- filtration unit 1 comprising a meshed membrane provided with a backwashing means for filtering roughly the waste water to be recovered from a car washing machine, an intermediate tank for storing the pre-filtrate, an ultrafiltration membrane module 3 of a hollow cellulose acetate fiber type for filtering the pre-filtrate by a closed-end flow system, an activated carbon treatment unit 4 for treating the filtrate from the module 3 with activated carbon and a treated water tank 5 for storing the water treated by the unit 4.