With the emphasis on the utilization of rich materials in the seabed in recent years, autonomous underwater vehicles (AUVs) are valued for technology to explore the oceans. Simultaneous operations of multiple-AUVs enable to investigate a wider area in a shorter time than the use of only a single AUV. During the survey, each AUV corrects the position error due to the inertial navigation system by the acoustic communication systems. All AUVs are required to be within the acoustic-communication available area. The formation control which can keep the desired relative positions becomes the mandatory function to explore the oceans efficiently while being within the acoustic-communication available area. In this study, a simple formation control-law based on a leader-follower algorithm was developed and validated through numerical simulations and sea trials. In the numerical simulations, waypoint tracking control-law and the proposed method were applied to the cruising-type AUVs and these control performances are compared. Sea trials using two cruising-type AUVs were carried out to evaluate the performance of the control-law.