Distribution map of the degree of coral has been developing by Manta method to use coral conservation activities. The manta method is a survey method that divers investigate the degree of coral and whitening states while being towed to a boat. The manta method makes great physical burden. Therefore, the authors think that this task can substitute an underwater robot. This underwater robot is desirable to be able to keep altitude above the seabed. Hence, the authors have been developing the altitude (its distance above the sea floor) keeping system for an underwater robot. Visual recognition of distant underwater objects is possible in the water with high transparency, for example, in the coastal sea area of Okinawa Prefecture, Japan. Therefore, the authors have been adopting the image-processing-based distance measurement method using the two laser beams and a monocular camera. It realizes to keep altitude of an underwater robot using such devices. The evaluation experiments of the altitude-keeping system are carried out in the pool.