Because moving objects usually moves on spatial networks, efficient trajectory index structures are required to gain good retrieval performance on their trajectories. However, there has been little research on trajectory index structure for spatial networks, like road networks. In this paper, we propose an efficient trajectory index structure for moving objects in Location-based Services (LBS). For this, we design our access scheme for efficiently dealing with the trajectories of moving objects on road networks. In addition, we provide both an insertion algorithm to store the initial information of moving object trajectories and one to store their segment information. We also provide a retrieval algorithm to find a set of moving objects whose trajectories match the segments of a query trajectory. Finally, we show that our trajectory access scheme achieves about one order of magnitude better retrieval performance than TB-tree.