Each task #1, #2 registers signal handlers belonging thereto in a signal-handler table through a handler registering section in an OS, and registers itself and the priority thereof in a task/handler priority table through a task registering section. When a signal is generated, a signal notifying section specifies a signal handler corresponding to the signal and the priority thereof by referring to the signal-handler table, and a task/handler selecting section selects a task or signal handler having a highest priority in the task/handler priority table. A task executing section executes the selected task and a signal-handler executing section executes the selected signal handler.