NOVELTY - A methane fermentation tank (1) is operated by alternately performing normal operation and sediment extraction operation. During normal operation, downward flow is produced toward bottom face (7) by rotating stirring apparatus in center of methane fermentation tank and upward flow is produced toward water surface on methane fermentation tank side. During sediment extraction operation, upward flow is produced toward water surface in center of methane fermentation tank by reversely rotating stirring apparatus with time of normal operation and downward flow is produced toward bottom face.    USE - Operating method of methane fermentation tank used in the fields of wastewater treatment and energy.    ADVANTAGE - Sediment can be efficiently removed from methane fermentation tank, methane fermentation tank can be continuously operated, and operating cost of the apparatus can be reduced.    DETAILED DESCRIPTION - A methane fermentation tank (1) is operated by alternately performing normal operation and sediment extraction operation. During normal operation, downward flow is produced toward bottom face (7) by rotating stirring apparatus in center of methane fermentation tank and upward flow is produced toward water surface on methane fermentation tank side. During sediment extraction operation, upward flow is produced toward water surface in center of methane fermentation tank by reversely rotating stirring apparatus with time of normal operation and downward flow is produced toward bottom face. Sediment (4) brought together in bottom face center part from sediment tube (6) is discharged out of methane fermentation tank. Methane fermentation tank has cylinder shape and comprises horizontal bottom face and center part which is provided with stirring apparatus (3) of screw-type or impeller-type which stirs to-be-processed liquid (2) and sediment tube positioned in bottom face center part. Angle of straight line which ties edge part and center part to upward or downward direction is 0.3-1.    DESCRIPTION OF DRAWING(S) - The drawing shows a conceptual diagram explaining stirring process of to-be-processed liquid in methane fermentation tank and demonstrating flow of to-be-processed liquid during reverse rotation of stirring apparatus.    Methane fermentation tank (1)    To-be-processed liquid (2)    Stirring apparatus (3)    Sediment (4)    Sediment tube (6)    Bottom face (7)