The object of the present invention is to develop an RNAi molecule that can permit the expression of a wild type gene or a desired mutated gene and can inhibit only the expression of a specific dominantly mutated gene selectively and efficiently and to provide a method for designing the RNAi molecule and an inhibitor of the expression of an dominantly mutated gene which contains the RNAi molecule as an active ingredient. Provided is an inhibitor of the expression of an dominantly mutated gene comprising an RNAi molecule which is designed so that in a dominantly mutated gene which can be transcribed into a transcript having a discontinuous connection point generated therein the base length from a nucleotide located at the 3 terminal of a sense strand region adjacent to the discontinuous connection point to the 3 terminal of the gene becomes a predetermined length.
