NOVELTY - A wet-slaking method for anaerobic thermophilic-digestion of organic waste is performed using a light emitting diode as a light source.    USE - As a wet-slaking method involving anaerobic thermophilic-digestion of organic waste (claimed) e.g. general organic sludge, sludge after waste water treatment obtained after fermentation, foodstuffs dreg, wastewater drain, and organic waste at sewage works, foodstuffs factory and purification tank; to produce anaerobic digestion dreg useful as organic fertilizer. The methane gas produced is useful as a boiler fuel, a micro gas turbine, a fuel of the after-modification/reformation fuel cell to hydrogen, and for digestion/sewage-gas electric power generation.    ADVANTAGE - The wet-slaking method results in rapid and efficient decomposition or degradation of organic waste, with efficient gasification i.e. improved gasification rate, by anaerobic digestion; as compared to that obtained by a prior art process of anaerobic digestion using dark filament. Thus results in increased methane production efficiency. The high temperature anaerobic digestion using the diode light source further suppresses the proliferation of photosynthetic bacteria that requires medium-temperature conditions. As the method remarkably reduces the proliferation of methane photosynthetic bacteria, a reduction of the biogas production due to reduction or restoration of the organic matter by photosynthetic bacteria proliferation does not occur. The activity of methane-fermentation property of microorganisms is also raised by light irradiation. The light emitting diode provides a conversion of electricity into direct light, and exhibits good energy efficiency characteristics without wastage; as compared to that obtained with incandescent lamp generator. The anaerobic digestion dreg produced, which is useful as organic fertilizer has low salt concentration, as compared to that obtained by aerobic compost, due to conversion of a solid phase part rather than a liquid part.