In a dynamic sensor network, mobile sensor nodes will continuously move around and frequently reconnect to other sensor nodes. However, current security protocols, which were designed for static networks only, cause a significantly large overhead when applied to dynamic networks. In this paper, we propose an efficient authenticated key agreement protocol that reduces the overall processes for the reauthentication of mobile nodes that have been authenticated once in the network. Along with a high efficiency in authentication of frequently reconnecting mobile nodes, we also introduce the "Neighbor Sink List" (NSL) that enables our protocol to be applicable to the irregularly distributed sensor networks in real environments. Our protocol realized computation that is about eight-time more energy efficient than that realized by PKI-based approaches and communication that is about 50% more energy efficient than that realized by approaches based on symmetric key cryptosystems.