A method for producing a maleimide-based copolymer, comprising an initial polymerization step, a middle polymerization step, a final polymerization step and an imidization step, where in the an initial polymerization step, a total amount of vinyl cyanide monomers to be added, 10 to 90% by mass of a total amount of aromatic vinyl monomers to be added, and 0 to 30% by mass of a total amount of unsaturated dicarboxylic anhydride monomers to be added are mixed to start copolymerization, in the middle polymerization step, 50 to 90% by mass of the remaining amount of the aromatic vinyl monomers used in the initial polymerization step and the remaining amount of the unsaturated dicarboxylic anhydride monomers used in the initial polymerization step are each separately or continuously added to the mixture to continue the copolymerization, in the final polymerization step, the remaining amount of the aromatic vinyl monomers used in the initial polymerization step and the middle polymerization step is added to the mixture to obtain aromatic vinyl-vinyl cyanide-unsaturated dicarboxylic anhydride copolymers, and in the imidization step, the obtained aromatic vinyl-vinyl cyanide-unsaturated dicarboxylic anhydride copolymers are imidized with ammonia or primary amine to obtain the maleimide-based copolymers, and wherein the maleimide-based copolymer as obtained by the method comprises 40 to 60% by mass of aromatic vinyl monomer units, 5 to 20% by mass of vinyl cyanide monomer units, and 35 to 50% by mass of maleimide-based monomer units, a tetrahydrofuran solution containing 4% by mass of the copolymer has a transmittance of 90% or more for light having a wavelength of 450 nm at an optical path length of 10 mm, obtained with the method specified in the description; and the amount of the residual maleimide-based monomers contained in the maleimide-based copolymer and determined according to the method defined in the description, is less than 300 ppm. where in the an initial polymerization step, a total amount of vinyl cyanide monomers to be added, 10 to 90% by mass of a total amount of aromatic vinyl monomers to be added, and 0 to 30% by mass of a total amount of unsaturated dicarboxylic anhydride monomers to be added are mixed to start copolymerization, in the middle polymerization step, 50 to 90% by mass of the remaining amount of the aromatic vinyl monomers used in the initial polymerization step and the remaining amount of the unsaturated dicarboxylic anhydride monomers used in the initial polymerization step are each separately or continuously added to the mixture to continue the copolymerization, in the final polymerization step, the remaining amount of the aromatic vinyl monomers used in the initial polymerization step and the middle polymerization step is added to the mixture to obtain aromatic vinyl-vinyl cyanide-unsaturated dicarboxylic anhydride copolymers, and in the imidization step, the obtained aromatic vinyl-vinyl cyanide-unsaturated dicarboxylic anhydride copolymers are imidized with ammonia or primary amine to obtain the maleimide-based copolymers, and wherein the maleimide-based copolymer as obtained by the method comprises 40 to 60% by mass of aromatic vinyl monomer units, 5 to 20% by mass of vinyl cyanide monomer units, and 35 to 50% by mass of maleimide-based monomer units, a tetrahydrofuran solution containing 4% by mass of the copolymer has a transmittance of 90% or more for light having a wavelength of 450 nm at an optical path length of 10 mm, obtained with the method specified in the description; and the amount of the residual maleimide-based monomers contained in the maleimide-based copolymer and determined according to the method defined in the description, is less than 300 ppm.