An object is to provide a method for producing an ever-larger molecular weight collagen-like polypeptide single strand. Another object is to provide a method for controlling a molecular weight of a product to be obtained in desired magnitude upon producing a collagen-like polypeptide single strand. A solution is a method for producing a polypeptide including a step for allowing a condensation reaction of peptide oligomers represented by any one of formulas (1) to (3) (SEQ ID No:1 to SEQ ID No:3), wherein the condensation reaction is carried out in an aqueous solvent containing a phosphate ion in the range of 0 M to less than 0.01 M in the presence of a condensing agent, or a condensing agent and a condensing auxiliary: H-(Pro-Y-Gly)n-OH  (1); H-(Y-Gly-Pro)n-OH  (2); and H-(Gly-Pro-Y)n-OH  (3); wherein, in formulas (1) to (3), Y is hydroxyproline or proline, and n is an integer from 1 to 10.