To provide a methane fermentation apparatus which removes ammonia efficiently, maintains the fermentation of organic substances for a long term and increases the recovery amount of methane gas.

The methane fermentation apparatus is provided with a tank 10 to store fermentation sludge B, a removal part 20 which introduces the fermentation gas generated by the methane fermentation of the fermentation sludge B in the tank 10 and removes an ammonia gas and carbon dioxide from the fermentation gas to circulate it to the tank 10, and a measuring part 30 to measure the pH in the tank 10. The removal part 20 removes carbon dioxide when the pH measured by the measuring part 30 is less than 7.8 and does not remove carbon dioxide when the pH exceeds 8.5.