In this paper, we propose a positioning method of multiple AUVs, Alternant Landmark Positioning, and show the development process of the proposed method. The key idea of the proposed method is alternating roles of the AUVs between Moving and Measuring Role (MMR) and Landmark Role (LR). AUVs in the MMR estimate their positions by particle filter based on the AUVs in the LR. On the other hand, AUVs in the LR keep their positions on the seabed to act as landmarks for the moving AUVs. By alternating their roles, the two groups of AUVs can observe in a wide area while maintaining a good position estimate. In order to develop the proposed method, two steps are assumed. The first step is developing mutual acoustic positioning method using an AUV and single seafloor station as a static benchmark for positioning. In this step, experiments were conducted in October 2011 using the testbed AUV Tri-Dog 1 and a static seafloor station at Kagoshima Bay in Japan. Based on seafloor pictures taken by the AUV at the same point, position estimation by mutual measurements was found to be within 0.5m after surveying for one hour. The second step is developing navigation method alternating two roles (MMR and LR) between two AUVs. Compared with initial step, the shortcoming of the proposed method is accumulation of positioning errors. In order to show the performance of proposed method, we examine the deviation from true states through the navigation simulation by two AUVs. We also formulated the accumulation of uncertainty caused by positioning errors. From the results between the simulation based on proposed method and formulation of error accumulation, the proposed method suppressed the deviation from true state by particle filter eliminating outlier measurements.