The relation between physiological indices and mental states has lately attracted considerable attention. This paper, therefore we try to examines the method of Respiratory Sinus Arrhythmia measurement and analysis method. We measured subjects' RSA during a pre and posttest, and examined the relation between the appearance-rate of RSA and the contents of the interview with the subjects. As a result of experiment, when subjects were sure of their answer to the test, the appearance rate of RSA showed a tendency to rise. When subjects were not sure of their answer to the test, the appearance rate of RSA showed a tendency to fall. From this result, we find that RSA can be an index of a certainty of the answer to a test.