A modified polysiloxane compound represented by formula (I): X(Y)n (I), where X represents a polymer block having a repeating unit represented by formula (II), Y represents a polymer block having a repeating unit represented by formula (III), and n represents 1 or 2, and having an X to Y weight ratio of 1/99≦X/Y≦90/10 and a number-average molecular weight of 1,000 to 100,000 is subjected to treat for elimination of a protecting group for a hydroxyl group by adding a non-aqueous solution containing hydrogen chloride in the presence of a non-aqueous solvent so that hydrogen chloride is 0.9 to 1.3 equivalents with respect to 1 equivalent of the repeating unit represented by formula (II).