NOVELTY - A methane fermentation tank (1) is operated by performing normal operation and sediment extraction alternately in which normal operation is carried out by producing downflow toward bottom face (7) by rotating stirring apparatus (3) of screw type or impeller type in the center of methane fermentation tank and producing upflow toward water surface on methane fermentation tank side, and sediment extraction is carried out by producing upflow toward the water surface in the center of methane fermentation tank by reversely rotating stirring apparatus of screw type or impeller type.    USE - Operation method of methane fermentation tank (claimed).    ADVANTAGE - Sediment is removed efficiently without adding other apparatus, operation of methane fermentation tank is continuous, and operation cost is reduced.    DETAILED DESCRIPTION - A methane fermentation tank (1) is operated by performing normal operation and sediment extraction alternately in which normal operation is carried out by producing downflow toward bottom face (7) by rotating stirring apparatus (3) of screw type or impeller type in the center of methane fermentation tank and producing upflow toward water surface on methane fermentation tank side, and sediment extraction is carried out by producing upflow toward the water surface in the center of methane fermentation tank by reversely rotating stirring apparatus of screw type or impeller type with the time of normal operation and producing downflow toward bottom face on the side surface of the tank. The sediment (4) brought together in the bottom face center part from the sediment extraction tube (6) are discharged out of the tank. The methane fermentation tank has a cylinder shape in which the angle of straight line connecting the center part and the edge part with the horizontal line is 0-0.3 degrees , center part is provided with stirring apparatus of screw type or impeller type which stirs to-be-processed liquid (2), and sediment extraction tube is arranged in the bottom face center part.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the stirring process of the to-be-processed liquid in a methane fermentation tank and the flow of the to-be-processed liquid during reverse rotation of a stirring apparatus.    Methane fermentation tank (1)    To-be-processed liquid (2)    Stirring apparatus (3)    Sediment (4)    Sediment extraction tube (6)    Bottom face (7)