The peptide in this invention is a peptide having affinity to gp120 represented by Formula (1): H-A1-A2-A3-A4-A5-R(SEQ ID No. 1) (in the formula, H means hydrogen, A1 is aspartic acid, lysine, valine, glutamic acid, glycine, asparagine, or tyrosine residue, A2 is valine, aspartic acid, tryptophan, lysine, phenylalanine, isoleucine, leucine, or tyrosine residue, A3 is lysine, valine, aspartic acid, arginine, alanine, or tryptophan residue, A4 is alanine, tryptophan, or glycine residue, A5 is glycine, alanine, valine, leucine, isoleucine, serine, threonine, methionine, asparagine, glutamine, histidine, lysine, arginine, phenylalanine, tryptophan, proline, or tyrosine residue, R is OH derived from carboxyl group or NH.sub.2 derived from acid amide group). The above peptide has an affinity to gp120 of the HIV envelope protein and is superior in stability.