An object of the invention is to provide an antimicrobial peptide having an amino acid sequence which is different from a peptide existing and functioning as an antimicrobial peptide in the natural world and is not based on the conventional developmental approach for an antimicrobial peptide-containing antimicrobial agent, and a polynucleotide coding for said peptide. Another object is to provide an antimicrobial agent which contains such an antimicrobial peptide. Namely, an antimicrobial peptide represented by a general formula (1) (Xa)n-S (1) wherein Xa of the formula (1) is a hydrophilic amino acid residue, n is an integer of from 1 to 6, two or more of the Xa may be the same or different from one another, S is a peptide represented by hydrophobic amino acid part-basic amino acid part-bridge part-basic amino acid part-hydrophobic amino acid part, and amino acid residue of the bridge part is selected from the group consisting of hydrophobic amino acids and neutral amino acids.