A method for manufacturing a powder for a magnetic core including at least a process of performing a siliconizing treatment on a surface of an iron powder (Ha) containing elemental carbon. In the process of siliconizing treatment, a powder (21a) containing at least a silicon dioxide is brought into contact with the surface of the iron powder (Ha), elemental silicon is detached from the silicon dioxide by heating the powder (21a) of silicon dioxide, and the siliconizing treatment is performed by causing the detached elemental silicon to permeate and diffuse into a surface layer of the iron powder (21a). The invention provides a method for manufacturing a powder for a magnetic core, by which loss reduction is achieved.