The purpose of the present invention is to provide a method for producing a very stable, cyclized mutant protein such that high cyclization efficiency is achieved while the number of amino acids added is minimal and the biological properties of an original protein are maintained. In view of conformational information about the original protein, secondary structure-free regions at N/C terminal portions are deleted. Then, a protein database is screened for proteins with secondary structures similar to those of N/C terminal residues of a secondary structure-forming portion after the deletion. The screening results are used to determine the amino acid length of a loop structure through which the N-terminus and the C-terminus of the secondary structure-forming portion of the original protein are to be connected. A cyclized mutant protein is finally produced having a loop structure with the determined amino acid length.