NOVELTY - The process for anaerobic fermentation of organic material (5) having high solid content (15-50 wt.%) in an airtight fermentation tank (10) in stratification conditions and in vertical movement from top to bottom in the fermentation tank under the action of the gravity, comprises brewing fermenting mass by alternating pressure rise of gas product (6) and sudden dropping of the pressure of the gas product. The alternation is obtained by rhythmical withdraw of gas product. The organic material is fermented at a temperature of 35-40 degrees C or 55-65 degrees C.    USE - The process is useful for anaerobic fermentation of organic material having high solid content (15-50 wt.%) in an airtight fermentation tank in stratification conditions, where the material vertically moves from top to bottom in the fermentation tank under the action of the gravity and is useful in agriculture, livestock and agri-food production.    ADVANTAGE - The process ensures simple, energy-efficient, economic and homogeneous anaerobic fermentation of organic material in the airtight fermentation tank in an environmental friendly manner.    DETAILED DESCRIPTION - The process for anaerobic fermentation of organic material (5) having high solid content (15-50 wt.%) in an airtight fermentation tank (10) in stratification conditions and in vertical movement from top to bottom in the fermentation tank under the action of the gravity, comprises brewing fermenting mass by alternating pressure rise of gas product (6) and sudden dropping of the pressure of the gas product. The alternation is obtained by rhythmical withdraw of gas product. The organic material is fermented at a temperature of 35-40 degrees C or 55-65 degrees C. In steady state, an amount of fermented material equal to the mass contained in the fermentation tank divided by a residence time of the fermenting mass in the tank is extracted every day in passes of extraction of the fermentation tank. The weight amount of organic material introduced in each pass of feed is such that it reduces the level or the weight of the quantity of material, which contains in the fermentation tank of defined nominal value. In steady state, the fermented material is continuously extracted and the organic material is continuously introduced at each time at a rate, which corresponds to a residence time of the fermenting mass in the fermentation tank to constantly maintain the weight or level of the fermenting mass in the fermentation tank. An INDEPENDENT CLAIM is included for an installation for anaerobic fermentation of organic material having high solid content.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a process for anaerobic fermentation of organic material.    Organic material (5)    Gas product (6)    Anaerobic fermentation tank (10)    Brewing system (19)    Extraction pipe. (171)