A hair cosmetic containing (A) a specified quaternary ammonium salt, (B) a dimethylpolysiloxane having a weight average molecular weight of 300 or more and 3,000 or less, and water, the hair cosmetic being used by applying on the hair and then rinsing away, wherein the content of the component (B) is 0.5 mass % or more and 60 mass % or less, and a receding contact angle (θR) of water droplet as measured by the expansion-contraction method with respect to the surface of a polyethylene plate which is applied with the hair cosmetic and then subjected to water washing is 82° or more and 110° or less. A treatment method of hair including (I) applying the hair cosmetic on the hair and spreading it over the entire hair, and (II) rinsing away the hair cosmetic with water from the entire hair.