A method for manufacturing a ferromagnetic-particle includes preparing a manufacturing apparatus including a single mode cavity that resonates with a microwave of a predetermined wavelength; a microwave oscillator electrically connected to the single mode cavity and configured to introduce the microwave of a predetermined wavelength into the single mode cavity; a pipe disposed to pass linearly through an inside of the single mode cavity, the pipe being formed of a dielectric material; and a pump configured to introduce, from one end of the pipe, an alkaline reaction liquid in which metal ions of a ferromagnetic metal and hydroxide ions are dissolved; and reacting the reaction liquid in the pipe, introduced by the pump, by introducing the microwave into the single mode cavity so as to generate the ferromagnetic-particle in the pipe.