PURPOSE: To reduce the amount of alkali used for pH regulation and to stably remove an organic substance by a simple operation at a low cost in the case of separating organic waste liquid into an acid producing process and a methane producing process and performing anaerobic treatment.

CONSTITUTION: Liquid 6 to be treated is introduced into an acid producing tank 1 to produce acid and thereafter introduced into a decarbonation tank 2 and air is aerated to perform decarbonation. Then, liquid 6 to be treated is introduced into a pH regulation tank 3 and alkali 22 is added to regulate pH and thereafter methane production is performed in a methane producing tank 4. At this time, the flow rate of liquid 6 to be treated is measured by a flowmeter 15 and a blower 12 is controlled in accordance with the measured value and over aeration is prevented.