After two recent reports of radioactive substances being detected in Tokyo's Setagaya Ward, the science ministry Wednesday acknowledged that it receives a number of similar reports of unapproved radioactive materials each year.

After the outbreak of the crisis at the Fukushima No. 1 nuclear power plant, such reports are likely to increase in the future, the ministry said, with more people, even those outside the disaster-hit regions, checking for highly radioactive materials near their homes.

"Because many radioactive materials had not been under control before the country set up a law, there could be an increasing number of such cases" reported, one official said.

Officials urge anyone who finds any unapproved radioactive materials to report them to the science ministry.

Under the 1957 law regulating the handling of radioactive materials, entities that use, sell, rent or scrap such materials must be approved by the science minister.

The law also stipulates that those who breach the regulations on the territory are punishable by up to three years in prison or fines of up to ¥1 million. However, materials found to have been handled without any approval before 1957 are not subject to punishment.

The ministry's website explains that many unregistered radioactive materials were reported before the recent cases in Setagaya Ward. Many cases were reported by hospitals that were not aware they had been in possession of the radioactive element radium-226 since before 1957. Research institutes also reported that they did not know that radioactive materials such as tritium and carbon-14 were being kept in their laboratories.

According to a 2002 report by the Nuclear Safety Commission, there were 136 cases in which radioactive materials were lost or stolen between 1958 and 2001.