PROBLEM TO BE SOLVED: To provide a thioesterase mutant and a method for producing lipid using the mutant.SOLUTION: The invention provides a method for producing lipid comprising: a step of introducing, into a host, a gene encoding a protein which has thioesterase activity to obtain a transformant； and a step of harvesting lipid from the obtained transformant. In a specific amino acid sequence derived from coconut, the protein has an amino acid sequence having at least one amino acid substitution of a selected amino acid, and has a thioesterase activity. The protein has an amino acid sequence in which one or some amino acids are deleted, substituted, inserted, or added, and has a thioesterase activity. The hosts are particularly Arabidopsis thaliana, cyanobacteria, and Escherichia coli, and the lipid is lauric acid or an ester thereof.
