To provide a methane fermentation method capable of improving a treatment efficiency of activated sludge by promoting denitrification of an activated sludge tank while optimally maintaining operation efficiency of a methane fermentation tank, and its system.

In the methane fermentation method, organic waste is slurried and charged into the methane fermentation tank 11, the waste is subjected to methane fermentation with anaerobic bacteria to be taken out as a digestive, and the digestive is treated in the activated sludge tank 12. In the method, the concentration of organic acid in the methane fermentation tank 11 is measured by an organic acid measuring device 17. When a measured value is a predetermined value or higher, a part of the slurry containing organic acid is taken out, and is subjected to solid/liquid separation with a separation membrane 14. The slurry is subjected to an organic acid addition process for adding solution containing organic acid to the activated sludge tank 12, thus maintaining the concentration of organic acid in the methane fermentation tank 11 bellow a predetermined value.