A new poly(carboxybetaine) whose main chain is composed of peptide bonds was synthesized from poly(gamma-methyl L-glutamate). The side chains of poly(gamma-methyl L-glutamate) were exchanged to carboxybetaine structures by three reactions. Our new poly(carboxybetaine) has 84% carboxybetaine structures in the side chains, as determined by H-1 NMR measurements. The presence of betaine structures in the side chains was confirmed by viscosity measurements.