This work presents a design and a field test result of a small USV system that has portability and wind tolerance for a lake bed depth mapping of volcanic crater lakes. The depth map of the crater lakes indicates an amount of water, crater wall caving, and volcanic upthrust, which is used for volcanic disaster prevention. However, todayfs depth surveillance is achieved by using a manned canoe on a lake in high-altitude, strong-winded, and restricted areas. This research is aiming to realize the autonomous Unmanned Surface Vehicle (USV) for volcanic crater lake surveillance in reducing risks to surveyors. In this research, the authors have developed the lightweight semisubmersible USV system with a high draft that has portability and wind tolerance. The result of a field test at Mt. Zao Okama Crater Lake is shown. A depth map of the north half of the Okama Crater Lake was autonomously measured by using the USV system.