To attain recycling of an alkaline waste liquid discharged when a bio-diesel fuel is manufactured from a waste vegetable oil by an alkali catalyst method, and to prevent acidification of a decomposition oil obtained by thermally decomposing polystyrene.

In the thermal decomposition method for producing the decomposition oil by thermally decomposing a polystyrene solution in which polystyrene is dissolved in a solvent, when the bio-diesel fuel is manufactured from the waste vegetable oil by the alkali catalyst method, the discharged alkaline waste liquid is mixed with the polystyrene solution, and the mixture solution is fed to a thermal decomposition apparatus to thermally decompose the polystyrene, thereby, acidification of the obtained decomposition oil is prevented.