The present invention relates to a parallel processing system by an OS for single processor capable of operating an OS for single processor and an existing application on a multiprocessor and achieving parallel processing by a multiprocessor with respect to the application, wherein the multiprocessor are logically divided into two groups, i.e., a first processor side and a second processor side, and units of work that are parallelizable within the application operating on the processors on the first processor side are controlled as new units of work on the processors on the second processor side.