Provided are a leave-on hair cosmetic composition, containing the following components (A), (B) and (C): (A) an organic dicarboxylic acid or salt thereof, (B) an organic solvent selected from aromatic alcohols, N-alkylpyrrolidones, alkylene carbonates, polypropylene glycols having a number average molecular weight of from 100 to 1000, lactones and cyclic ketones, and (C) an organopolysiloxane having an organopolysiloxane segment and a poly(N-acylalkyleneimine) segment having a recurring unit represented by the following formula (1): (wherein, R1: H, C1-22 alkyl, cycloalkyl, aralkyl or aryl, and n: 2 or 3), wherein the latter segment has been bonded to at least one silicon atom of the former segment via a hetero-atom-containing alkylene group; having the organopolysiloxane segment and the poly(N-acylalkyleneimine)segment at a weight ratio of from 98/2 to 40/60; and having a weight average molecular weight of from 50,000 to 500,000; and a hair treating method with the composition. The hair cosmetic composition of the invention can essentially improve the hair quality, give excellent luster, manageability, setting property to the hair, and also give excellent flexible hair feel and elasticity.