To provide a method for concentrating membrane filtered washing wastewater, capable of effectively concentrating washing wastewater generated at the time of filtering treatment of raw water containing soluble manganese by a separation membrane.

In the filtering treatment of a suspended substance in raw water containing soluble manganese by the separation membrane, the pH of raw water and the concentration of dissolved oxygen in the raw water are controlled to convert the soluble manganese to insoluble manganese dioxide, and particles of formed manganese dioxide are filtered along with the suspended substance by the separation membrane and washing wastewater of the separation membrane is filtered by a granular filter medium to concentrate the washing wastewater containing the suspended substance.