In this session, we will introduce the efforts to meet high security requirements in the Private Cloud environment “Flava.” We will begin with an overview of the Private Cloud, followed by a retrospective look at how LINE and Yahoo! JAPAN have historically built applications with strict security demands. Then, we will explain the new security approach developed after the merger of the two companies. In addition to discussing concrete security measures, we will also explore a more abstract perspective on security—how we conceptualize it and apply it to actual cloud systems.