The present invention provides a PCB decomposing apparatus capable of decomposing PCB efficiently. The PCB decomposing apparatus comprises a PCB extracting container for extracting PCB from a PCB-containing material with an organic solvent. A distilling tower provided on the downstream side of the PCB extracting container distills the organic solvent to separate the PCB. A primary reactor provided on the downstream side of the distilling tower includes a nozzle for supplying aqueous Na2CO3 solution and a nozzle for supplying an oxidizing agent into the container. A liquid cyclone provided on the downstream side of the primary reactor can remove Na2CO3 having a large particle size. A secondary reactor formed by a long pipe is connected to the liquid cyclone. A gas-liquid separator provided on the further downstream side of the secondary reactor can separate a gas from a solution. A film separator separates treated water into purified water and a concentrated solution, and the purified water is supplied as circulating water to the upstream side of a pressure pump.