A hair treatment agent containing 3 to 30% by mass of one or more selected from the group consisting of metaphosphoric acid and salts thereof, polyphosphoric acid and salts thereof, phosphonic acid and salts thereof, and phosphonic acid metal ion chelating agents. Further, the pH of the hair treatment agent is in the range of 3.5 to 10.0. Further, the hair treatment method comprises: (1) applying the hair treatment agent to the hair； (2) placing the hair in a coated state to sufficiently apply the hair treatment agent to the hair； (3) washing the hair and washing off the hair treatment agent； 4) A step of drying and making the hair straight.一種毛髮處理劑，含有3～30質量%的由偏磷酸及其鹽類、聚磷酸及其鹽類、膦酸及其鹽類、膦酸系的金屬離子螯合劑選擇的一種或二種以上，且該毛髮處理劑的pH在3.5～10.0的範圍。又，毛髮處理方法包含：（1）於毛髮塗布該毛髮處理劑；（2）在塗布狀態下放置毛髮使毛髮處理劑充分作用於毛髮；（3）水洗毛髮並洗掉毛髮處理劑；及（4）乾燥及使毛髮成為直髮狀的步驟。
