[Problems] To provide a method of producing a lipid, containing enhancing productivity of medium chain fatty acids or the lipid containing these medium chain fatty acids as components.[Means to solve] A method of producing a lipid, containing the steps of: culturing a transformant in which a gene encoding any one of the following proteins (A) to (C) is introduced into a host, and collecting a lipid from the cultured product: (A) a protein consisting of the amino acid sequence of the 91st to 348th positions set forth in SEQ ID NO: 1; (B) a protein consisting of an amino acid sequence having 80% or more identity with the amino acid sequence of the 91st to 348th positions set forth in SEQ ID NO: 1, and having acyl-ACP thioesterase activity; and (C) a protein containing the amino acid sequence of the protein (A) or (B), and having acyl-ACP thioesterase activity.