A metal-containing particle aggregate of an embodiment of the present invention includes a plurality of core-shell particles. Each of the core-shell particles includes: a core portion that contains at least one magnetic metal element selected from the first group consisting of Fe, Co, and Ni, and at least one metal element selected from the second group consisting of Mg, Al, Si, Ca, Zr, Ti, Hf, Zn, Mn, rare-earth elements, Ba, and Sr; and a shell layer that includes a carbon-containing material layer and an oxide layer that covers at least part of the core portion and includes at least one metal element that belongs to the second group and is contained in the core portion.