In a parallel processing system by an OS for single processors which operates an OS and an existing application for single processors on a multiprocessor to realize parallel processing by the multiprocessor with respect to the application, a processor on a first processor side receives a request for activating or stopping a processor from a unit of work on any of the processors and controls a power supply management device of the OS for single processors to conduct activation or stop of the requested processor, while the processor requested to be activated or stop executes processing necessary for the activation or stop based on a notification from the first processor side.