PURPOSE: To cope with the variation in the amount of biomass charged in a methane fermentation tank with latitude, by biochemically solubilizing the biomass having high water-content.

CONSTITUTION: The biomass raw material 1 having high water-content is fed through the line 1, washed with water in the washing step 3, and roughly crushed in the crushing step 4. A part of the crushed mass is sent through the line 5 to the drying step 6 and dried to a water content of 40W60% e.g. in the sun. The rest part of the mass is sent together with the dried mass through the line 7 to the biochemical solubilizing step 9, in which the raw material and the (semi)dried material is solubilized biochemically at about room temperature under strong acidic conditions or at about 7pH under high-load conditions. The solubilized raw material is stored in the storage, taken out of the storage at need, and subjected to the methane fermentation in the methane fermentation step 11.