Problem to be Solved: To provide a new microorganism capable of producing a polyhydroxyalkanoate (PHA), a PHA synthase gene, an expression cassette including the gene, a vector including the expression cassette, a transformant transformed by the vector, a polypeptide having PHA synthase activity, a method for producing a PHA synthase and a method for producing a PHA. Solution: The new microorganism is capable of producing a polyhydroxyalkanoate comprising a 16S rRNA gene whose polynucleotide sequence shows 99% or more homology to a polynucleotide sequence represented by SEQ ID No: 1, having an optimum temperature of an activity temperature range for the growth and PHA production of the microorganism of at least 45.degree. C. and being capable of growing at a pH range from 6 to 10.