Disclosed is a flame retardant processing method characterized by comprising the following steps: a radiation processing step of irradiating a cellulosic fiber material with a radioactive ray; a phosphorus treatment step of adding a radical-polymerizable phosphorus-containing compound to the cellulosic fiber material; and an amine treatment step of adding an amine compound to the cellulosic fibermaterial. Also disclosed is a cellulosic fiber material imparted with flame retardancy, which is produced by the method.