The present invention relates to a gene coding for a copolyester-synthesizing enzyme, a microorganism which utilizes the gene for the fermentative synthesis of a polyester, and a method of producing a polyester with the aid of the microorganism. More particularly, the present invention relates to a gene which functions in a host organism and, by an enzyme, synthesizes a plastic-like polymer degradable under the action of microorganisms in the natural environment (the soil, river or sea). The present invention also, more particularly, relates to a transformant derived from the host organism by transformation with the gene and having an improved ability to fermentatively synthesize a plastic-like polymer, and a method of producing a copolyester with the aid of the transformant.