An aspect of the present invention relates to a method of manufacturing hexagonal ferrite powder, which comprises preparing a hexagonal ferrite precursor-containing water-based solution by stirring and mixing a reaction solution which comprises an iron salt, an alkaline earth metal salt, and a base in a reaction tank, and removing the hexagonal ferrite precursor-containing water-based solution that has been prepared from the reaction tank and continuously feeding the hexagonal ferrite precursor-containing water-based solution into a reaction flow passage while conducting heating and pressurizing to converting the hexagonal ferrite precursor to hexagonal ferrite, wherein the preparation of the hexagonal ferrite precursor-containing water-based solution comprises a continual feed period during which feeding of the iron salt, the alkaline earth metal salt, and the base into a reaction tank which comprises a prereaction solution in which an iron salt and a base are not both present is continuously or intermittently continued, during the continual feed period, at least a base is fed over a feed passage separate from feeding of the iron salt and the alkaline earth metal salt, and an amount of at least one from among the iron salt, the alkaline earth metal salt, and the base that is fed per unit time during the continual feed period is controlled and/or acid is added to the reaction solution in the reaction tank so that a pH of the reaction solution within the reaction tank during the continual feed period falls within a range relative to a pHbefore, a pH of the prereaction solution, of pHbefore−2≦pH≦pHbefore+2.