A method for purifying a fluid such as an exhaust gas and waste water, which comprises providing one or more of static mixers on the flow path of the fluid, and mixing and reacting the fluid with a depurative such as ozone in the static mixer to purify the fluid. The method for purifying a fluid allows the purification of a fluid such as an exhaust gas and waste water with significantly good efficiency, and also allows the simplification of an apparatus for use in the purification and the markedly reduction of the cost required for the purification.