There is provided a method for efficiently produce hydroxypropyl methyl cellulose acetate succinate (HPMCAS) having excellent flowability, where acetic acid in a reaction product mixture subjected to a wash and recovery step can be reduced. More specifically, there is provided a method for producing HPMCAS including an esterification step of esterifying hydroxypropyl methyl cellulose (HPMC) with acetic anhydride and succinic anhydride in acetic acid as a solvent to obtain a reaction product solution containing HPMCAS; a water addition step of adding water to the reaction product solution to obtain a water-added reaction product solution; an acetic acid removal step of removing at least a portion of both the solvent acetic acid and acetic acid derived from the acetic anhydride from the water-added reaction product solution to obtain a mixture having an acetic acid content reduced; and a wash and recovery step of washing the mixture and recovering the HPMCAS.