An index to express the degree of anxiety/expectation quantitatively is presented. The new index is defined based on the probability of occurrence of the undesirable/desirable matter by introducing the concept of the information entropy and the weight of value of the undesirable/desirable matter. The definition has no factor of feeling and bases only on the change of amount of information entropy about the occurrence/disappearance of the undesirable/desirable matter. Additionally, the way of application and the usefulness of the newly defined index are made clear by applying it to several close-at-hand problems. Particularly, the new index shows its usefulness clearly when it is applied to make decisions with security.