A problem to be solved by the present invention is to provide a double-stranded nucleic acid complex having a reduced toxicity whose effectiveness is not impaired. Provided is a double-stranded nucleic acid complex comprising a first nucleic acid strand and a second nucleic acid strand, wherein: said first nucleic acid strand is capable of hybridizing to at least part of a target gene or a transcription product thereof, and has an antisense effect on said target gene or transcription product thereof, said second nucleic acid strand comprises a base sequence complementary to said first nucleic acid strand, and said first nucleic acid strand and/or said second nucleic acid strand comprises at least one bridged non-natural nucleoside represented by formula (I) or formula (II) (wherein R represents a hydrogen atom or a methyl group).