The present invention relates to a method for producing middle-chain-length polyhydroxyalkanoate (MCL-PHA) using a maoC gene. The producing method of MCL-PHA according to the present invention comprises the steps of transforming a microorganism with the maoC gene to give a transformant, the microorganism being deleted of a fadB gene and containing a PHA synthase gene; culturing the transformant in medium containing a C6-10 carbon source; and obtaining PHA consisting of monomers with 6–10 carbon atoms. When the maoC gene whose function has not yet been established is used according to the present invention, high quality PHA with a higher number of carbon atoms than the prior PHA can be produced at a higher efficiency.