To protect the secrecy of digital contents by providing a in network a server, which stores a monitor program with high interruption priority constituted as a real-time operating system utilizing a microkernel.

To a network 50, servers 51 to 54 and clients 55 to 58 are connected. Each of the servers 51 to 51 is stored with the basic element of an operating system as a microkernel and an application element or digital contents as a subsystem. A management server 51 grasps the use of digital contents and digital content management programs by the clients 55 to 58. A monitor process automatically monitors without any request from the clients 55 to 58, illegal use of digital contents by periodically interrupting processes that the clients 55 to 58 are executing and issues an alarm or forcibly stops the illegal use if the illegal use is detected.