The authors are currently experiencing an explosion of cloud technologies. However, a cloud service administrator may be an untrustworthy third party. Therefore, companies dealing with confidential information cannot use a public cloud. In this paper, they propose a method for preventing the leakage of private information on a cloud schedule service. In this method, even a cloud administrator or a hacker who steals a cloud service login key cannot read the true schedule because the schedule date is altered and schedule content is encrypted. Consequently users can safely use a public cloud schedule service with this method. They also evaluated the method's performance using an actual alteration program on Google server.