A method of treating biomass material to achieve based on a reaction due to organic degradation by microbe in the biomass material by contact to oxygen, a volume reduction or carbonization of one type or more of biomass material which is selected among food scraps, livestock excreta, agricultural waste products, marine waste products and forest waste products in a pressurizable and heatable container, comprising: placing the biomass material in the container; setting an initial condition of the container in order to generate an exothermic reaction of the biomass material after placing the biomass material, the initial condition including all of (a) an atmosphere in the container having oxygen, (b) a temperature range in the container being equal to or greater than 55 degree Celsius and equal to or less than 80 degree Celsius, (c) a pressure range in the container being equal to or greater than 0.2 MPa (2 atmospheric pressure) and equal to or less than 1.5 MPa (15 atmospheric pressure) and (d) carbon monoxide concentration being equal to or greater than 100 ppm; and maintaining a continuation condition after the temperature of the biomass material exceeds 80 degree Celsius in order to spontaneously increase temperature of the biomass material over at least 150 degree Celsius, the continuation condition including all of: (i) the atmosphere in the container having oxygen; (ii) the pressure range in the container being equal to or greater than 0.2 MPa (2 atmospheric pressure) and equal to or less than 1.5 MPa (15 atmospheric pressure); and (iii) a carbon monoxide concentration in the container being equal to or greater than 100 ppm, wherein the container comprises a drain valve, a heating device, and a pressure device, and the drain valve drains water on the bottom of the container regularly or irregularly along with increasing the temperature of the biomass material. placing the biomass material in the container; setting an initial condition of the container in order to generate an exothermic reaction of the biomass material after placing the biomass material, the initial condition including all of (a) an atmosphere in the container having oxygen, (b) a temperature range in the container being equal to or greater than 55 degree Celsius and equal to or less than 80 degree Celsius, (c) a pressure range in the container being equal to or greater than 0.2 MPa (2 atmospheric pressure) and equal to or less than 1.5 MPa (15 atmospheric pressure) and (d) carbon monoxide concentration being equal to or greater than 100 ppm; and maintaining a continuation condition after the temperature of the biomass material exceeds 80 degree Celsius in order to spontaneously increase temperature of the biomass material over at least 150 degree Celsius, the continuation condition including all of: (i) the atmosphere in the container having oxygen; (ii) the pressure range in the container being equal to or greater than 0.2 MPa (2 atmospheric pressure) and equal to or less than 1.5 MPa (15 atmospheric pressure); and (iii) a carbon monoxide concentration in the container being equal to or greater than 100 ppm, (i) the atmosphere in the container having oxygen; (ii) the pressure range in the container being equal to or greater than 0.2 MPa (2 atmospheric pressure) and equal to or less than 1.5 MPa (15 atmospheric pressure); and (iii) a carbon monoxide concentration in the container being equal to or greater than 100 ppm,