This hair treatment agent is obtained by blending leucine with a modified peptide that has a group represented by formula (I) and/or a salt thereof as a side chain group. It is preferable that the content of leucine is 0.1 part by mass or more relative to 100 parts by mass of the modified peptide. It is also preferable that a modified peptide wherein the main chain is composed of keratin is blended as the above-mentioned modified peptide. This hair treatment agent is a hair care agent or a hair styling agent. -S-S-(CH2)n-COOH (I) (In formula (I), n is 1 or 2.)
