A system for processing supercritical and subcritical fluid capable of bringing the inside of at least one processing container (1) formed in a flow passage into a supercritical or subcritical high pressure field, wherein thermal operation is applied to process fluid to apply thermal expansion to the fluid to produce a pressure difference between the processing container (1) and the outside, whereby a specified temperature and the high pressure field suitable for the processing of the supercritical or subcritical fluid can be provided in the processing container (1).