The present invention addresses the problem of providing a method for producing microparticles having a controlled crystallite diameter. At least two types of fluids to be processed; i.e., a raw material fluid containing a substance to be separated, and a separating fluid containing a substance for causing separation of the substance to be separated, are mixed in a thin film fluid formed between at least two processing surfaces which are approachably and separably disposed facing each other. At least one processing surface rotates relative to the other, and the substance to be separated giving a controlled crystallite diameter is caused to separate. At this time, specific conditions related to the fluid to be processed introduced between the processing surfaces are varied to control the crystallite diameter of the substance to be separated. The specific conditions are at least two conditions selected from the group consisting of: the type of substance to be separated included in the raw material fluid and/ or substance included in the separating fluid; the concentration of the substance to be separated included in the raw material fluid and/ or substance included in the separating fluid; the pH of the raw material fluid and / or separating material fluid; the introduction temperature of the raw material fluid and/ or separating fluid; and the introduction velocity of the raw material fluid and/ or separating fluid.