The present invention provides a cationic polyamino acid suitable for a carrier that can form a stable complex with a nucleic acid under a physiological condition and release the nucleic acid in cells suitably. The cationic polyamino acid can associate with a nucleic acid and includes a cationic amino acid residue having a cationic group in a side chain and a hydrophobic amino acid residue having a hydrophobic group in a side chain. The cationic polyamino acid includes 1 to 20 units of the cationic amino acid residue and is represented by the following formula (1). The meaning of each symbol in the formula is as shown in the description.