Cyclic peptides which comprise, as a constituent chain or chains thereof, one or two amino acid sequences selected from the amino acid sequence Asn-Val-Ser-Glu-Ala-Asp-Asp-Arg-Tyr-Ile and the amino acid sequence Arg-Ser-Gln-Lys-Glu-Gly-Leu-His-Tyr-Thr, and AIDS vaccines containing at least one of the cyclic peptides as an active ingredient. From the in vivo absorption and antibody expression viewpoint, a substituent group is preferably bound to at least one active group selected from among the carboxyl, amino and hydroxyl groups contained in the cyclic peptides. The cyclic peptides can neutralize the second receptors which the HIV-1 virus utiliizes in the infection of humans therewith.