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 like absence of infrastructure, dynamically changing topologies and channels of nodes. For securing networks or hosts, many Intrusion detection systems (IDSs) have been proposed. However, most existing intrusion detection techniques developed on the WLANs and wired networks are not suitable for ad hoc networks. One of the most considerable properties in a MANET is energy constraint because batteries carried by each node have limited power supply, processing power is limited, which turn limits services and applications that can be supported by each node. Especially Denial of service (DoS) attacks, which consume a large amount of batteries inefficiently and unexpectedly are a critical issue in MANET. In this paper, we suggest an effective Intrusion Detection System (IDS) level in a MANET against DoS attacks considering to reduce energy consumption and save expected costs for operating IDS. Moreover, in order to lessen energy consumption, the algorithm should be simple. The results illustrate that our proposed algorithm can use limited resources effectively with maintaining appropriate security level in a MANET.