PURPOSE: To continue a processing in a dangerous area, and to prevent a system down by setting a flag to indicate that the processing of a process is moved to the processing of the dangerous area, and blocking a signal for moving the process to an interruption processing.

CONSTITUTION: At the time of executing the processing in the dangerous area, a process 12A preliminarily declares the start of the processing in the dangerous area. At the time of declaring the start of the processing in the dangerous area, the process 12A sets ON the critical flag of a process management table 13 assigned to its own process 12A. Next, the set of the signals to be blocked is designated as factors, and a 'sigsetmask' system control is issued. When the system control is issued, the signal to the process 12A is blocked, and the processing can be continued. When the processing in the dangerous area is ended, the critical flag of the process management table 13 assigned to its own process 12A is turned OFF.