A method for manufacturing an antibody-drug conjugate composition comprising (i) a step for reacting an antibody with a reducing agent in a buffer solution to reduce the interchain disulfides, and (ii) a step for reacting a drug-linker intermediate with the antibody having a thiol group obtained in step (i), the manufacturing method being characterized in that: the reaction temperature in step (i) is -10°C to 10°C； the average number of coupled drugs in the manufactured antibody-drug conjugate composition is 3.5 to 4.5； and the proportion of antibody-drug conjugates in which four drug-linkers are coupled to a heavy-light interchain thiol is 50% or higher. An antibody-drug conjugate composition in which the average number of coupled drugs is 3.5 to 4.5, and the proportion of antibody-drug conjugates in which four drug-linkers are coupled to a heavy-light interchain thiol is 50% or higher.
