A method for producing magnetic microparticles, wherein at least two fluids are used, wherein: at least one kind of the fluids is a fluid containing at least one kind of a magnetic raw material, at least one other kind of the fluids is a fluid containing at least one kind of a magnetic microparticles-separating agent, the method comprising: introducing the at least two fluids between two processing surfaces (1,2) arranged opposite to each other so as to be able to approach to and separate from each other, at least one of which rotates relative to the other, generating a force to move in a direction of separating a second processing surface of the processing surfaces from a first processing surface of the processing surfaces by a pressure of the fluids, generating a surface-approaching pressure generated by a surface-approaching pressure imparting mechanism to exert in a direction of approaching the first processing surface and the second processing surface, maintaining a minute space between the first processing surface and the second processing surface by the balance between the force to move in the direction of separating the second processing surface from the first processing surface and the surface-approaching pressure generated by the surface-approaching pressure imparting mechanism; forming a thin film fluid by passing the at least two fluids between the first processing surface and the second processing surface maintained in the minute space, and wherein the magnetic raw material and the magnetic microparticles-separating agent are reacted in the thin film fluid, whereby magnetic microparticles are separated in the thin film fluid to obtain the magnetic microparticles; and wherein the magnetic microparticles have a volume-average particle size of 0.5 nm to 1000 nm. at least one kind of the fluids is a fluid containing at least one kind of a magnetic raw material, at least one other kind of the fluids is a fluid containing at least one kind of a magnetic microparticles-separating agent, introducing the at least two fluids between two processing surfaces (1,2) arranged opposite to each other so as to be able to approach to and separate from each other, at least one of which rotates relative to the other, generating a force to move in a direction of separating a second processing surface of the processing surfaces from a first processing surface of the processing surfaces by a pressure of the fluids, generating a surface-approaching pressure generated by a surface-approaching pressure imparting mechanism to exert in a direction of approaching the first processing surface and the second processing surface, maintaining a minute space between the first processing surface and the second processing surface by the balance between the force to move in the direction of separating the second processing surface from the first processing surface and the surface-approaching pressure generated by the surface-approaching pressure imparting mechanism; forming a thin film fluid by passing the at least two fluids between the first processing surface and the second processing surface maintained in the minute space, and wherein the magnetic raw material and the magnetic microparticles-separating agent are reacted in the thin film fluid,