a gene expression cassette comprising an aeromonas caviae - origin polyhydroxyalkanoic acid synthase gene having a mutation introduced thereinto and a promoter and a terminator acting in a yeast ; a transformant constructed by transferring the above gene expression cassette into a yeast ; and a process for producing a polyester using the above transformant . thus , it becomes possible to efficiently produce a copolymerized polyester having a high biodegradability and excellent properties by copolymerizing 3 - hydroxyalkanoic acids in a yeast .