When an active volcano erupts, a restricted area is typically set according to the eruption level. However, it is very important to observe eruption products inside of this area to predict the timing and scale of volcanic hazards, such as debris flows. Therefore, we propose a robotic observation system for active volcanoes that is composed of a multi-rotor unmanned aerial vehicle (UAV) and a mobile ground robot. To deliver the ground robot safely from the multi-rotor UAV to the ground, we implemented a sky-crane mechanism and confirmed the feasibility of the mechanism theoretically. In this paper, we introduce our volcano observation scenario as well as the observation system and sky-crane mechanism we have developed. Finally, we report on a field test conducted at Mount Asama in September 2013.