The evaluation of security technology has been concerned with how secure a system is from the theoretical and performance viewpoints. On the other hand, the majority of computer users have not been sure about how secure the systems and services which they use really are. Not so much work has been done on how well a user interface incorporates users' subjective feelings such as the sense of security. We look into this problem and propose an initial model of evaluating security systems in terms of the sense of security. The factors include knowledge, trust and impression. We conducted an experiment with users of a secure memo submission system, the under-the-door system, to demonstrate empirically how those factors influence the sense of security.