Effective method for manufacturing antibody-drug conjugate

The present invention is: a method for manufacturing an antibody-drug conjugate in which a drug linker represented by formula (1) (A in the formula representing an antibody binding site) and an antibody are bonded by a thioether bond, the manufacturing method characterized by including (i) a step for reducing an antibody with a reducing agent, (ii) a step for reacting a drug linker intermediate with the antibody reduced in step (i), (iii) a step for adding a reagent having a thiol group and reacting with the drug linker intermediate remaining from step (ii), and (iv) a step for then removing a byproduct derived from the drug linker intermediate by ultrafiltration using a buffer solution containing a salt comprising a strong acid and a strong base; and a method for manufacturing a pharmaceutical composition including the aforementioned antibody-drug conjugate.