Retardation film, and optical compensation layer, optical compensation polarizing plate, liquid crystal display device and organic el display device each using said retardation film

A retardation film that has excellent wavelength dispersion characteristics, excellent in-plane retardation and excellent film thickness is achieved by using a polymer material which is formed of at least one cellulose derivative having a specific alkoxyl group substitution degree (D 1 ) and a specific 2-naphthoyl group substitution degree (D 2 ), and which has a specific total 2-naphthoyl group substitution degree (D 3 ).