To obtain the subject new oligonucleotide having a complementarity to an adult T-cell leukemia virus gene and inhibiting the manifestation by combining with this gene, comprising an HTLV-1 anti-sense oligonucleotide as an antibody containing a specific base sequence.

This new oligonucleotide has a partial sequence comprising at least 15 bases in the base sequence of the formula. The oligonucleotide is useful as an anti-HTLV-1 anti-sense oligonucleotide having a complementarity to a specific base sequence of a gene of an adult T-cell leukemia virus (HTLV-1) and capable of inhibiting the manifestation of the viral gene by utilizing the complementarity. The oligonucleotide is obtained by synthesizing various oligonucleotides targeting a vicinity of an initiation codon of a main protein of HTLV-1, a splicing binding part, a binding part of a protein, etc., and searching a base sequence having high anti-virus effect by using a virus antigen manifestation-suppressing test.