There are provided HPMCAS (hypromellose acetate succinate) having such a property that a solution of the HPMCAS in a solvent has a controlled viscosity； and a method for producing the HPMCAS. More specifically, provided are HPMCAS having such property that a solution of 10 parts by weight of the HPMCAS in 100 parts by weight of a mixed solvent having a weight ratio of methylene chloride to methanol of 1:1 has a viscosity at 20°C of 135 mPa·s or less； and a method for producing the HPMCAS including an esterification step of adding acetic anhydride and succinic anhydride to a solution of hypromellose in glacial acetic acid in the presence of sodium acetate to obtain a reaction product mixture, wherein the succinic anhydride is added intermittently, and a precipitation step of mixing the reaction product mixture with water to precipitate the HPMCAS.
