PURPOSE: To obtain a compound excellent in film formability, etc., by regulating the degree of hydroxypropylation of cellulose and the viscosity of a solution of the cellulose in an aqueous NaOH solution under given conditions each to a spicified volve.

CONSTITUTION: This compound has a degree of hydroxypropylation of 0.2-0.9 and a 2wt.% solution thereof in an 8wt.% aqueous NaOH solution has a viscosity (at 25°C) of 1,000cP or higher. It can be obtained preferably by reacting cellulose (e.g. linter) with 0.8-1.5mol of an alkali (e.g. NaOH) per mol of the anhydroglucose units in a medium consisting of 85-92wt.% t-butyl alcohol and water, the amount of the medium being 10-20 times by weight the amount of the cellulose, and reacting the resulting alkali cellulose with 1.1-5.0mol of propylene oxide per mol of the anhydroglucose units at 50-70°C for 2-5hr.