A method of producing a lipid, comprising steps of introducing a gene that encodes the following protein (a) or (b) into a host, and thereby obtaining a transformant, and collecting a lipid from the transformant: (a) A protein comprising an amino acid sequence of the 112nd to 414th amino acids set forth in SEQ ID NO: 1 in which an amino acid corresponding to the 209th amino acid set forth in SEQ ID NO: 1 is substituted from tryptophan to threonine, glutamic acid or alanine, and having thioesterase activity, and (b) A protein comprising an amino acid sequence of the protein (a) in which one to several amino acids other than the amino acid corresponding to the 209th amino acid set forth in SEQ ID NO: 1 are deleted, substituted, inserted or added, and having thioesterase activity.