NOVELTY - Method (M1) for preparing microorganism carrying support, involves (a) contacting culture solution with electroconductive support and microbial population which contains support target microorganisms group to the electroconductive support, controlling the electric potential of electroconductive support until the support target microorganism reaches optimal range, and activating the electroconductive support containing support target microorganism group predominantly.    USE - The method (M1) is useful for preparing microorganism carrying support.    ADVANTAGE - The microorganism carrying support exhibits excellent processing capacity under high load condition.    DETAILED DESCRIPTION - Method for preparing microorganism carrying support, involves (a) contacting culture solution with electroconductive support and microbial population which contains support target microorganisms group to the electroconductive support, controlling the electric potential of electroconductive support until the support target microorganism reaches optimal range, and activating the electroconductive support containing support target microorganism group predominantly, (b) contacting hydrophobic electroconductive support which can support microorganism with methane fermentation liquid containing organic substrate controlling the electric potential of the electroconductive support at which a reduction reaction may be produced or until it reaches + 0.3 V on basis of silver-silver chloride electrode potential and activating the microorganism supported on the electroconductive support or (c) contacting hydrophobic electroconductive support which can support microorganism with culture solution containing substrate of methane, controlling the electric potential of the electroconductive support at which a reduction reaction may be produced and activating the microorganism supported on the electroconductive support. INDEPENDENT CLAIMS are included for the following:    (1) methane fermentation method (M2) involves performing the above-cited steps; and    (2) method (M3) for processing substance, involves contacting microorganism carrying support with liquid containing to-be-processed object, and controlling the electric potential of microorganism carrying support until the support target microorganisms reaches optimal range.