For user registration, feature of a file system of a computer of a particular user is extracted, and a licenser generates key data characteristic of the extracted feature and provides the key data to the particular user. At the time of installation of particular software to be protected or actual use (startup) of the software, the user's computer reads the current feature of its file system, and an authorization test is performed for determining whether or not to permit use of the software by the user's computer, on the basis of presence/absence of identity or similarity between a current feature of the file system and the feature of the file system initially evaluated at the time of the user registration. On the condition that it has been determined that the use of the software by the user's computer can be permitted, the key data is updated to match the current feature of the file system. There may be provided a plurality of determination gates at a plurality of locations within the software to perform the authorization test at the plurality of locations. In this case, the use of the software by the user's computer may be inhibited if the authorization test has made a negative determination at any one of the determination gates, so as to reliably forestall unfair alteration of the software.