To provide a peptide having algicidal activity to algae causing red water that causes discoloration of sea weeds.

The invention relates to the peptide with a specific amino acid sequence. The peptide couples with cells of algae which causes red water, and kills the cells. The peptide comprises C-end region of a serine protease outside cell of Pseudoaltermonas sp., and easily expressed by an expression system such as Esherichia coli expression system.