A technique for prohibiting access to a computer having a security function when a security device is illegally removed from the computer. The security device is a hardware component that constitutes a part of a security function for a computer. When the security device is illegally removed from the computer, access to the computer is prohibited. Data indicating that the security device is attached to the computer are stored in a nonvolatile memory. Then, when a specific event, such as a power-ON event, is used as a trigger, the procedure for prohibiting the access to the computer is initiated. Following this, based on the data stored in the memory, it is detected that the security device was once attached to the computer and that it has now been removed. If currently the security device is not attached to the computer, although it was attached before, the access to the computer is prohibited.