NOVELTY - The method comprises closing biogas fermenters (2) loaded with fresh biomass (6), flushing the closed biogas fermenter with carbon dioxide-containing exhaust gas over exhaust gas line and biogas inlet (12) of the biogas fermenter to be startedup until the carbon dioxide concentration in the biogas fermenter reaches a lower limit and/or until the methane concentration in the biogas fermenter reaches a lower limit, and supplying gas mixture from the biogas fermenter to be startedup to a gas processing device (20).    USE - Method for reducing methane slip during startup of biogas fermenter in a biogas plant useful for producing biogas.    ADVANTAGE - The method ensures simple and efficient reduction of methane slip during startup of biogas fermenter in a biogas plant thus enabling production of biogas with high quality.    DETAILED DESCRIPTION - The method comprises closing biogas fermenters (2) loaded with fresh biomass (6), flushing the closed biogas fermenter with carbon dioxide-containing exhaust gas over exhaust gas line and biogas inlet (12) of the biogas fermenter to be startedup until the carbon dioxide concentration in the biogas fermenter reaches a lower limit and/or until the methane concentration in the biogas fermenter reaches a lower limit, supplying gas mixture from the biogas fermenter to be startedup to a gas processing device (20), when the lower limit for the carbon dioxide concentration and the methane concentration is exceeded, increasing the methane content in the gas mixture through partial separation of non-methane components in the gas processing device, measuring the methane concentration in the gas mixture to be processed by a measuring device, recycling the gas mixture to be processed into the biogas fermenter, when the measured methane concentration lies under a pre-determined upper limit, and supplying the gas mixture to be processed over the biogas supply line in the common biogas line, when the measured methane concentration lies over the upper limit. INDEPENDENT CLAIMS are included for:    (1) a method for reducing methane slip during shutdown of biogas fermenters in a biogas plant; and    (2) a biogas plant for producing biogas.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of a biogas fermenter.    Fermenter (2)    Biomass (6)    Biogas inlet (12)    Gas processing device (20)    Gas conveying device. (24)