An information processing system having a mechanism wherein mutual verification is performed between tasks and a computer operating system at the time of activating tasks, thereby judging the validity of tasks. The operating system evaluates a key which the task holds at the time of requesting service of the operating system, and permits execution of services only in the event that the operating system itself has the same key. Management information is written by the operating system to a secure memory queue list based on a security level, and a key is written to a buffer. The operating system deciphers the contents of the buffer wherein the received contents exist, with the key, based on the security level, and copies this onto a buffer on the mail receiving task.