NOVELTY - A to-be-fermented raw material is poured from a feed pipe into a fermenting pipeline to be fermented to generate methane. Methane is collected in fermenting regions, and extrudes biogas slurry. The biogas slurry in the fermenting pipeline is transferred into a water-pressure unit, and when the air pressure of methane in any one fermenting region is reduced, the biogas slurry in adjacent fermenting region is transferred to the fermenting region and the biogas slurry in the water-pressure unit is transferred back to the fermenting pipeline, and thus the biogas slurry is circulated.    USE - Method for generating methane (claimed).    DETAILED DESCRIPTION - A to-be-fermented raw material is poured from a feed pipe into a fermenting pipeline to be fermented to generate methane. Methane is collected in fermenting regions formed between the feed/discharge pipe and adjacent gas baffle plates or between adjacent gas baffle plates, where methane occupies the space in the fermenting region and extrudes biogas slurry to generate convection and flow to the feed pipe, the discharge pipe and a bacteria distributing device. Along with elevation of the liquid level of the biogas slurry stored in the feed pipe, the discharge pipe and the bacteria distributing device, the biogas slurry in the fermenting pipeline is transferred into a water-pressure unit through a drain hole of the discharge pipe, and when the liquid level of the biogas slurry stored in the water-pressure unit reaches the lowest position of a drain pipe of the bacteria distributing device, the biogas slurry is transferred into the water-pressure unit through the drain pipe of the bacteria distributing device, and when the air pressure of methane stored in any one fermenting region is reduced, the biogas slurry in adjacent fermenting region is transferred to the fermenting region and the biogas slurry stored in the water-pressure unit is transferred back unidirectionally to the fermenting pipeline through the drain hole of the feed pipe and transferred back to the fermenting pipeline through the drain hole of the discharge pipe and the drain pipe of the bacteria distributing device, such that the methane space reduced in the fermenting region is compensated. Thus, the biogas slurry is circulated.