PROBLEM TO BE SOLVED: To provide a device for manufacturing a ferromagnetic fine particle, capable of manufacturing ferromagnetic fine particles in a state having an almost uniform particle diameter even if fluid flowing through a reactor is not heated.SOLUTION: A method for manufacturing a ferromagnetic fine particle includes: a tubular reactor 11 having an inner diameter of 0.01 to 2.0 mm which defines a reaction passage and an aqueous solution-supplying mechanism 20 in which a first aqueous solution 24a containing metal ions of ferromagnetic metal dissolved therein and a second alkaline aqueous solution 24b containing hydroxide ions dissolved therein are joined to each other to be supplied from one end side of the reactor 11 to the inside. The aqueous solution-supplying mechanism 20 includes a first passage 21a in which the first aqueous solution 24a is adjusted to a prescribed flow rate and guided a second passage 21b in which the second aqueous solution 24b is adjusted to the prescribed flow rate and guided and an aqueous solution confluent part 22 in which the first passage 21a and the second passage 21b are joined to each other.
