To provide a method capable of preventing unauthorized installation of software and unauthorized starting of it in an information processing terminal machine such as a personal computer.

In this software unauthorized installation/start preventing method, the following steps are carried out. In steps S15 and S16, use of the software in one information processing terminal machine is assumed as one user, and one predetermined non-contact data carrier, to which a unique ID is given, is associated with one user, while an encryption ID as encrypted unique ID is previously stored in a user memory area in the non-contact data carrier, so that the unique ID and the encryption ID of the non-contact data carrier are acquired before installation. In a step S17, a decoded unique ID is obtained by decoding the encryption unique ID. In a step S19, the unique ID is compared with the decoded unique ID. In a step S20, the software is installed when the unique ID and the decoded unique ID are discorded. In a step S21, the software is not installed when accordance is not given between the unique ID and the decoded unique ID.