The present invention is directed to a copolymer containing a 3-hydroxybutyrate (3HB) unit and a 3-hydroxyhexanoate (3HHx) unit, a three-component copolymer containing at least a 3-hydroxybutyrate (3HB) unit and a 3-hydroxyhexanoate (3HHx) unit, and a four-component copolymer containing at least a 3-hydroxybutyrate (3HB) unit and a 3-hydroxyhexanoate (3HHx) unit. The use of a microorganism of the genus Aeromonas according to the present invention makes it possible to produce a wide variety of plastic materials ranging from rigid plastics to elastic plastics by selecting copolymer components and adjusting their composition.