On a parallel processing system by an OS for single processors which operates, on a multiprocessor, an OS for single processors and an existing application to realize parallel processing by the multiprocessor with respect to the application, each processor includes a communication proxy unit which transfers data between tasks spreading over the processors by proxy and the communication proxy unit on a processor in which a task on a transmission side operates holds information about an address, on a processor, of a task on a reception side to receive data transferred from the task on the transmission side as proxy for the task on the reception side.