A mobile ad hoc network (MANET) is a collection of wireless mobile nodes forming a temporary network without any established infrastructure. MANETs are generally more vulnerable to security threats than ﬁxed wired network due to its inherent characteristics such as absence of infrastructure, dynamically changing topologies. The selection of IDS operating node is one of critical issues because of energy limited feature of a MANET. In this paper, we propose a cost-eﬀective IDS operating node selection scheme by solving cost minimization problem in a MANET. The results illustrate that our proposed algorithm can reduce the total cost while maintaining appropriate security level in a MANET.