The purpose of the present invention is to develop and provide a nucleic acid molecule that can specifically and efficiently inhibit the activity of a target RNAi molecule and can be produced safely at a low cost. Provided is a nucleic acid molecule for inhibiting the activity of a target RNAi molecule. The nucleic acid molecule comprises a single-stranded nucleic acid moiety that contains one unmodified DNA region composed of a nucleotide sequence completely or sufficiently complementary to a nucleotide sequence of a functional strand having the activity in the target RNAi molecule and a double-stranded nucleic acid moiety to be linked to at least one of the 5′-end and the 3′-end of the single-stranded nucleic acid moiety.