There are many kinds of control systems, which have been used in industry area, such as Process Automation (PA) and Factory Automation (FA). They have numerous sensors and actuators called field devices in their networks, and are introducing Internet Protocol (IP) as network technology. Network security is necessary to make the most of IP?s capability. The authors have previously studied a security mechanism which can satisfy the restrictions required by control systems and a secure autonomous bootstrap mechanism which is based on the security mechanism. This paper 1 shows an extension of the bootstrap mechanism to support multicast security. The discussion in this paper excludes security of multicast routing protocols and admission control protocols, e.g. IGMP and MLD, because the subject of this paper and those protocols are orthogonal.