Abstract: Fluid containing organic substances, which has not processed yet, is fed to a plurality of cylinders, using a slurry pump. A free piston is included in each cylinder, the fluid which has not yet been processed is introduced into one of two chambers in the cylinder partitioned by the piston, and processed high-pressure fluid which has been processed is introduced into the other one of the two chamber in the cylinder. The piston pressurizes the fluid with which the cylinder has been filled up. A pressure holding valve is provided in a processed high-pressure fluid system to pressurize an organic substance processing system, and this pressure holding valve suppresses changes in the pressure in the processed high-pressure fluid system.