Monitoring of volcanic phenomena close to active volcanic vents and inside active craters is needed to predict change of volcanic activities and to understand dynamics of volcanic eruptions. In order to carry out safe volcanic monitoring, we have developed a prototype of a mobile sensor for volcanic observation "HOMURA" which is a new robotic system that has been designed to observe volcanic phenomena inside active volcanic craters. HOMURA is a small unmanned ground vehicle (approx. 780×560×300mm in dimension and 10kg in weight) with six wheels driven by electric motors and it is operated by wireless remote control at a distance of more than 1km. Data measured by some sensors in HOMURA are sent to the base station in real time. Materials of the vehicle body and wheels are aluminum with 2mm thick and plywood with 9mm thick, respectively. HOMURA can climb up and down a rough surface with slope angle of 30 degree. In addition, HOMURA does not readily become undrivable even in overturning during climbing because it has a unique body shape with a horizontal symmetry plane. HOMURA can be made and transported to mission fields at small costs. These allow us to make a new vehicle even if HOMURA should be lost by accident during missions and promptly to explore a sudden volcanic event by HOMURA. In test campaigns at Aso volcano and Izu-Oshima volcano, we confirmed that HOMURA has planned abilities on moving on rough surfaces and wireless communication.