Method for decomposing polyhydroxyalkanoic acid, and microorganism preparation

Provided are: a method for decomposing a polyhydroxyalkanoic acid using an anaerobic microorganism flora that can decompose a polyhydroxyalkanoic acid with high efficiency to thereby convert the polyhydroxyalkanoic acid to a biogas containing methane as the main component; and a microorganism preparation which can be used as seed sludge for use in a methane fermentation process. A polyhydroxyalkanoic acid can be decomposed with a microorganism flora containing both a methane-producing archaeon as mentioned in item (A) and a bacterium as mentioned in item (B). (A) A methane-producing archaeon belonging to the genus Methanosarcina , or a single-rod-shaped hydrogen-utilizing methane-producing archaeon. (B) A bacterium which is classified into any one division selected from the division Acidobacteria , the division Bacteroidetes , the division Chloroflexi , the division Firmicutes , the division Nitrospirae , the division Proteobacteria , the division Synergistetes , the division Thermotogae and an unclassified division, and which can decompose a polyhydroxyalkanoic acid.