The present invention provides a method for stabilizing an anti-dementia drug in a pharmaceutical composition containing the anti-dementia drug and a high molecular weight basic substance by adding a high molecular weight acidic substance to said pharmaceutical composition. Further, the present invention provides a pharmaceutical composition containing an anti-dementia drug and a high molecular basic substance in which a high molecular weight acidic substance is contained for stabilizing the anti-dementia drug. Furthermore, the present invention provides a method for manufacturing a pharmaceutical composition which comprises steps wherein a solution or suspension containing a high molecular weight acidic substance is added to a mixture of an anti-dementia drug and a high molecular weight basic substance for the sake of stabilizing the anti-dementia drug.