A cellulosic substance composition includes: a cellulosic substance having a branched structure or cyclic structure-containing aliphatic acyl group (A) having from 4 to 30 carbon atoms, the aliphatic acyl group (A) being represented by formula (1): ##STR00001## wherein R.sup.11, R.sup.12 and R.sup.13 each independently represents a hydrogen atom or an alkyl group, provided that at least two of R.sup.11, R.sup.12 and R.sup.13 are an alkyl group; R.sup.11, R.sup.12 and R.sup.13 may each independently have a substituent; at least two of R.sup.11, R.sup.12 and R.sup.13 may be connected to each other to form a ring; and a portion of * represents a bond to a cellulose skeleton of the cellulosic substance.