An information processing system that can reduce its power consumption by means of robust power controlling even upon occurrence of an interruption/exception processing. If it is found that there is no task set in the ready state as a result of watching the number of tasks set in the ready state by the RTOS or ready state watching task, the system controls the RTOS or ready state watching task so as to lower the power while the current active task controls the power according to the preset WCET of each application slice. On the contrary, if there is any task set in the ready state, the system controls so as to raise the power and the current active task comes to control the power according to the virtual WCET that is earlier than the WCET of each application slice. And, if there is no task set in the ready state and there is no current active task, the system controls so that the RTOS or both of the ready state watching task and sleep task lower the power. This is why the present invention can reduce the power consumption of the subject information processing system.