To provide: a hair treatment agent that achieves a high hair lengthening effect without the use of reducing agents and oxidizing agents that greatly damage hair and a hair treatment method that makes it possible to lengthen hair more effectively and easily than in the prior art without excessively damaging hair. A hair treatment agent comprising (a) 10-25 mass% of glyoxylic acid and (b) 0.5-3 mass% of a guanidine salt and/or urea, said hair treatment agent having a pH in the range of 1.0-2.5. In addition, a hair treatment method according to the present invention comprises a step (1) in which hair is coated with the hair treatment agent, a step (2) in which the hair is left to stand, a step (3) in which the hair is washed with water, a step (4) in which the hair is dried, and a step (5) in which the hair is straightened with a hair styling iron.本發明提供一種不使用會對毛髮造成較大損傷之還原劑或氧化劑而實現高度之毛髮伸長效果之毛髮處理劑、以及不會過度地損傷毛髮而可較先前更有效且更容易地使毛髮伸長之毛髮處理方法。毛髮處理劑含有(a)10~25質量%之乙醛酸與(b)0.05~3質量%之胍鹽及/或脲,且具有1.0~2.5之範圍之pH值。又,毛髮處理方法包含如下步驟:(1)將上述毛髮處理劑塗佈於毛髮;(2)放置毛髮;(3)對毛髮進行水洗;(4)使毛髮乾燥;及(5)利用整髮用熨斗矯正毛髮。
