To provide a method of preventing the contamination by exhaust air/waste water in a vacuum drying device, capable of minimizing the contamination of living environment caused by matters discharged from the device while minimizing the equipment cost and the operation cost.

An alkaline mixing means is attached to a vacuum drying tank for mixing a specific amount of an alkaline additive agent coexisiting with an acid substance easily scattered in a processed material to modify the same into the hardly evaporatable property, into the processed material. An alkaline substance neutralizing means is mounted in an air discharge passage connected to a discharge port of the vacuum drying tank, for neutralizing and capturing a gaseous alkaline component generated from the processed material. Preferably, pH of the processed material is adjusted approximately 10. A temperature in the air discharge passage may be controlled to prevent the dilution of involatile acid solution as the alkaline substance neutralizing means, by water vapor.