To provide an apparatus for treating wastewater which can efficiently treat wastewater having a high SS concentration and alkali concentration with a simple configuration, reducing the running cost.

In the apparatus for treating wastewater 1, firstly, membrane filtration of an SS component in wastewater is carried out by a membrane filtration device 3 with a membrane filtration unit 32. The membrane filtration unit 32 is an immersion type, and therefore the wastewater having a high SS concentration can be efficiently treated. Since it is treated by the membrane filtration device 3, chemicals or the like are not required and a cost can be significantly reduced. Filtered water is colorless and transparent, and alkaline. Accordingly, only an alkaline component in the filtered water needs to be removed by adsorption. Since an ion exchange device 5 only requires to maintain a weak acid cation exchange resin, the construction of the ion exchange device 5 can be simplified and also an equipment cost can also be reduced. The regeneration of resin only requires to exchange the weak acid cation resin, and therefore a running cost using the chemicals or the like can also be significantly reduced.