The present invention aims to provide a nucleic acid compound that hardly forms non-Watson-Crick base pairs, and an oligonucleotide containing the nucleic acid compound and showing reduced non-specific binding with nucleic acids other than the target nucleic acid. The nucleic acid compound according to the present invention is characterized in that the 2-position carbonyl group of the pyrimidine base is functionally converted (X1 and X2 are each independently S or Se), and that the 2'-position and the 4'-position are bridged in a particular structure. The oligonucleotide according to the present invention is characterized in that at least one of thymidine and uridine is the nucleic acid compound.
