To control the film contamination of a membrane separation module, and to reduce the cleaning frequency in an anaerobic treating tank which performs the anaerobic treating of an organic wastewater, and a wastewater treatment apparatus which performs the aerobic treating of the treatment liquid of the anaerobic treating tank and solid-liquid separates by a membrane separation module to obtain a treated water.

The treatment apparatus of organic wastewater is characterized as follows. In the anaerobic treating tank 1 which performs the anaerobic treating of an organic wastewater, and a wastewater treatment apparatus which performs the aerobic treating of the treatment liquid of the anaerobic treating tank 1 by an aerobic treating tank 2, and solid-liquid separates by a membrane separation module 4 to obtain a treated water; the membrane separation module 4 is a tank external type membrane separation module.