Cosmic-ray muon radiography has been used to probe the internal structure of volcanoes. To overcome limitations on constructing an observation station, we developed a small-sized readout system with power consumption low enough to be powered by a small solar-power system. The system can be placed near a volcano and can detect cosmic-ray muons that penetrate the volcano and subsequently generate an angular distribution of the muons. The system can be connected to a PC placed remotely from the station, using a wired or wireless network. The PC can show the angular distribution of the muons on a web browser. We used this system to monitor an actual volcano. The obtained clear image of the volcano indicates that the performance of the readout system is sufficient for cosmic-ray muon radiography.