There is provided a filter for filtering gas or liquid, wherein a constituent material of the filter contains a protein that absorbs iron, radium, or heavy metal (except iron), the constituent material is an iron-containing fiber body 4, the iron-containing fiber body 4 is a fiber body including iron or a fiber body 41 on which iron is supported, and the protein is ferritin or hemosiderin and bound to iron of the iron-containing fiber body 4.