The Japan Meteorological Agency's Coordinating Committee for Predictions of Volcanic Eruption identified 47 out of 110 active volcanoes across the country that have the possibility of erupting within about a century and therefore need stronger surveillance and supervision.
The 47 volcanoes are under 24-hour watch by the agency and institutions such as volcanic monitoring and information centres in three district meteorological observatories - Sapporo, Sendai and Fukuoka.
Observing such factors as rises and falls in the numbers of volcanic earthquakes and the expansion of mountains by using a seismograph and the Global Positioning System, they will report to prefectural governments on information to beware of from a disaster prevention perspective.
Local governments have set up volcano disaster management councils and applied a system of Volcanic Alert Levels to 30 of the 47 volcanoes.
The system, which stipulates such points as how climbers should behave according to each mountain's current alert level, came to include Mt. Ontake in March 2008.
The alert level for Mt. Ontake was 1 before the eruption on Saturday, and has been 3 since the eruption. Other volcanoes at Level 3 are Sakurajima island and Kuchinoerabujima island, both in Kagoshima Prefecture.
The alert levels for Mt. Kusatsu-Shirane, lying on the border of Gunma and Nagano prefectures and Mt. Aso in Kumamoto Prefecture rose to 2 from 1 this year.
To introduce the alert level system, local governments are required not only to launch volcano disaster management councils but also to stipulate how the government would handle volcanic activities in different levels.
Local governments related to 17 volcanoes have not yet set up the councils and so the alert system has not been applied to them, as most of them have not erupted in recent years.
The meteorological agency is encouraging those local governments to launch the councils. But it is undeniable that volcano disaster measures in the nation lag even further behind than measures against landslides and tsunami which have also been criticised for being insufficient.