A computer system having a verification system for verifying whether or not disks are being used in an unauthorized manner comprises a plurality of computers each capable of being loaded with a disk storing application programs, and an authentication server capable of being connected with the computers via a network, wherein each computer has a unique device ID, each disk has a unique disk ID, and the server has a user database for accumulating the device ID when using the computer and the disk ID when using the disk, such that data is accumulated in the user database in pairs of disk ID and device ID. In the event of using the disk for the second time or subsequent times, data for the disk ID and data for the device ID are transmitted from the computer to the host server, thereby verifying at the host server that the usage is valid.