A method of processing substrates, comprising: loading a substrate into a process chamber; supplying a supercritical fluid, that is a process fluid under a supercritical state, into the process chamber, chemicals separated from the substrate and the supercritical fluid being mixed into a supercritical mixture in the process chamber; and gradually decreasing a chemical concentration of the supercritical mixture by alternately repeating a pressure drop mode and a supplemental mode such that the supercritical mixture partially flows out from the process chamber at the pressure drop mode when an inner pressure of the process chamber reaches a first pressure and the supercritical fluid turbulently flows into the process chamber at the supplemental mode when the inner pressure of the process chamber reaches a second pressure that is smaller than the first pressure and over a supercritical pressure of the process fluid.