a process for regenerating unreacted vinyl chloride monomers is provided which has the step of compressing by means of an compressor an unreacted vinyl chloride monomer recovered from a process of vinyl chloride polymer production ; compressing the same in contact with a lubricating oil fed into the compressor . in this process , the lubricating oil contains a polymerization inhibitor . hence , even when this lubricating oil is circulated and reused as a lubricating oil after its separation from the unreacted vinyl chloride monomer , any vinyl chloride monomer remaining in the lubricating oil can effectively be kept from polymerizing in lubricating - oil circulation lines .