Because an information system is used in different activities simultaneously today, we have to analyze usages of the system in the existing activities and to-be usages in an intended activity together. Especially, security aspects should be carefully analyzed because existing activities are not always secure. We propose a security requirements analysis method for resolving this problem. To take both existing and intended activities into account together, we integrate them on the basis of the unification of common actors. To explore possible attacks under integrated activities, we enumerate achievable attacks on the basis of the possible means in each actor with the help of security knowledge. To avoid or mitigate the attacks and to achieve fundamental goals, we disable some means or narrow down the means to be monitored with the help of propositional logic formulae. Through case studies on insurance business, we illustrated our idea.