A method for producing an antibody-drug conjugate composition, comprising: (i)a step of reacting an antibody with a reducing agent in a buffer to reduce interchain disulfides, and (ii)a step of reacting drug linker intermediates with the antibody having thiol groups obtained in the step (i), wherein the reaction temperature in the step (i) is -10°C to 10°C, and the average number of bound drugs in the produced antibody-drug conjugate composition is 3.5 to 4.5, and the content of antibody-drug conjugates in which four drug linkers are bound to heavy-light interchain thiols, in the produced antibody-drug conjugate composition is 50% or more； and an antibody-drug conjugate composition, wherein the content of antibody-drug conjugates wherein the average number of bound drugs is 3.5 to 4.5, and the content of antibody-drug conjugates in which four drug linkers are bound to heavy-light interchain thiols, is 50% or more.
