A method for producing isolatable oxide microparticles or hydroxide microparticles using an apparatus that processes a fluid between processing surfaces of processing members that are arranged opposite each other so as to be able to approach to or separate from each other and such that at least one can rotate relative to the other. Using an apparatus that processes a fluid between processing surfaces (1 and 2) of processing members (10 and 20) that are arranged opposite each other so as to be able to approach to or separate from each other and such that at least one can rotate relative to the other, at least two fluids are mixed and oxide microparticles or hydroxide microparticles are separated, said two fluids including: a fluid containing a microparticle raw material solution comprising a microparticle raw material mixed into a solvent, and a fluid containing a microparticle-separation solution. Immediately thereafter, the following are mixed to obtain isolatable oxide microparticles or hydroxide microparticles: a fluid containing the separated oxide microparticles or hydroxide microparticles; and a fluid containing a microparticle-treatment-substance-containing solution that contains a microparticle-treatment substance that adjusts the dispersibility of the separated oxide microparticles or hydroxide microparticles.