To provide a fermentation treatment system for organic waste capable of efficiently and satisfactorily performing fermentation treatment using fermentation material to be treated as material to be returned.

This fermentation treatment system is provided with a fermentation tank 1 supported in a freely rotatable operation manner around a horizontal axis, an air supplying mechanism 3 for supplying air into the fermentation tank 1 and an air discharge mechanism 4 for discharging air from the fermentation tank 1. The fermentation tank 1 comprises a main body container 5 provided with a supplying/ discharging opening 6 that is freely openable and closable and a material to be returned storage container 7 which is smaller than the main body container 5 in volume and communicatingly and projectingly connected to the main body container 5 at a part different from the supplying/discharging opening 6. The opening 6 is configured to have the same container shape as that of the container 7, both the opening 6 and the container 7 are connected to the main body container 5 in a V-shape, configuring the fermentation tank 1 with its cross section in a Y-shape. In the air discharge mechanism 4, a deodorizing chamber 22 filled with a deodorant 23 is formed between the container 7 and the opening 6 to discharge air outside the fermentation tank 1 through the deodorizing chamber 22.