Antisense oligonucleotide sequences which enable the measurement of the distribution and structure of antisense oligonucleotide drugs in the body, with lapse of time, and a method of detecting these sequences are provided. The antisense chains have a natural or non-natural nucleotide or peptide nucleic acid as a structural unit in which carbon atoms and nitrogen atoms are substituted by ¹³C and ¹⁵N, respectively, and the antisense chains can be detected by nuclear magnetic resonance spectroscopy (NMR) such as ¹⁵N—¹H or ¹³C—¹H hetero nuclear multiple quantum coherence spectroscopy.