PURPOSE: To provide a method for decomposing polyclorinated biphenyl that is safe and efficient, and allows decomposed substances to be easily dumped.

CONSTITUTION: In the decomposition of polyclorinated biphenyl group, a polyclorinated biphenyl in a fluid to be processed is decomposed using a chemical decomposition process, and a dechlorination reaction is stopped at a point when all amount of a high-chlorinated polyclorinated biphenyl to be processed is dechlorinated to be a low chlorinated polyclorinated biphenyl group with mono-, di- or trichloride and the low chlorinated polyclorinated biphenyl group with mono-, di- or trichloride is decomposed using microorganism. To perform a microorganism decomposition, a product or a fractional distillation at 150 to 350°C of the product resulted from a chemical decomposition process and a water solution containing nutrient salts and polyclorinated biphenyl group decomposing fungus are mixed under aerobic condition.