Abstract: In the present invention, a sequence segment conforming to the following rules (a) to (d) is searched from the base sequences of a target gene of RNA interference and, based on the search results, siRNA capable of causing RNAi is designed, synthesized, etc.: (a) The 3? end base is adenine, thymine, or uracil, (b) The 5? end base is guanine or cytosine, (c) A 7-base sequence from the 3? end is rich in one or more types of bases selected from the group consisting of adenine, thymine, and uracil, and (d) The number of bases is within a range that allows RNA interference to occur without causing cytotoxicity.