5.5 to 9.0, preferably from 7.0 to 8.5, but the possibility is not excluded that a pH condition is set in a range other than the above described range depending on the most suitable pH and pH stability of a PHA synthesizing enzyme to be used. [0142] The reaction temperature is set as appropriate depending on the property of the PHA synthesizing enzyme to be used, but may be set normally at 4 to 50