An organic wastewater treatment apparatus is operated by switching between a first operation mode and a second operation mode. The first operation mode includes supplying organic wastewater to a dual-use tank and performing an anaerobic treatment process therein, performing a nitrification-denitrification process in a membrane bioreactor treatment tank, and extracting membrane-permeated water from a membrane separation device arranged in the membrane bioreactor treatment tank as treated water. The second operation mode includes supplying the organic wastewater to the dual-use tank and performing an aerobic treatment process therein, supplying mixed liquid containing activated sludge from the dual-use tank to both of the membrane bioreactor treatment tank and a sedimentation tank, performing a nitrification-denitrification process in the membrane bioreactor treatment tank, extracting membrane-permeated water from the membrane separation device arranged in the membrane bioreactor treatment tank as treated water, and also extracting solid-liquid separated liquid from the sedimentation tank as treated water.