Provided is a method of preventing digital content stored in a universal serial bus (USB) mass storage (UMS) device from being distributed without restriction. In the method, registration data is encrypted in order to allow a UMS device to decrypt and use digital content using time information only within a predetermined term of validity. Accordingly, even if encrypted registration data of a UMS device is disclosed, it is possible to make it difficult for unauthorized devices to store the registration data, thereby preventing content of the UMS device from being distributed without restriction.