Provided is a producing method for metal microparticles for which the particle diameter is controlled. At least two types of fluids to be processed are used, and at least one of those fluids to be processed is a metallic solution wherein a metal and/or metallic compound is dissolved in a solvent. Of the fluids to be processed other than the above, at least one is a reducing agent fluid that includes a reducing agent. The fluids to be processed are mixed in a thin film fluid created between at least two processing surfaces (1, 2) which are disposed to face each other and can be brought closer or separated, at least one face rotating relative to the other, and metal microparticles for which the particle diameter is controlled are precipitated out. At that time, the particle diameter of the metal microparticles is controlled by varying particular conditions for at least one of the metallic solution and the reducing agent fluid introduced between the processing surfaces (1, 2). The particular conditions are at least one type selected from a set comprising the introduction rate for the metallic solution and/or reducing agent fluid and the pH of the metallic solution and/or the reducing agent fluid.