Process for producing a copolyester of desired quality at low cost with high productivity, preferably a process for producing P(3HB-co-3HH) of 4 mol % or more 3HH content with a productivity as high as 40 g/L or more. In particular, a process for producing a copolyester of 3HB and 3HH comprising culturing a microorganism with the use of, as a carbon source, a fat or oil containing lauric acid as a constituent fatty acid under such conditions that phosphorus as a source of nutrition is limited.