When an active volcano erupts, it is important to have visual images of the area to be able to forecast debris floods and/or pyroclastic flows. However, restricted zones are usually established within a radius of a few kilometers of the crater because of the direct danger to humans. Therefore, we propose an observation system based on a teleoperated mobile robot that is controlled using radio communication during volcanic activity. To evaluate the system, we conducted field tests using a 3G cellular phone inside certain volcanoes. During the experiments, we faced several dangerous situations where the robot stopped all motion because of the weakness of the 3G signal. To solve this problem, we developed a hybrid communication system with multiple robots that employs two radio communication links. In the proposed system, each robot is controlled via 3G communication signals. However, if any of the robots lose the 3G link, the control signal is relayed by another neighboring robot using a local communication link. In this paper, we explain the system, introduce our newly designed robots, and present results of our operation tests.