Disclosed is a cellulose diacetate comprising a 6-position highly acetylated cellulose diacetate having a 6-percent viscosity of 40 to 600 mPas, and having a total degree of acetyl substitution of DSt and a degree of acetyl substitution at the 6-position of DS6, wherein DSt and DS6 satisfy following Conditions (1) and (2): 2.0.ltoreq.DSt<2.6 (1) 0.400.gtoreq.(DS6/DSt).gtoreq.0.531-0.088.times.DSt (2) The cellulose diacetate is usable typically as a starting material for cellulose acylates of different acyl groups, which have a high total degree of acyl substitution, have superior optical properties, and usable typically as photographic materials and optical materials.