[en] Environmental radionuclide monitoring systems set up around a nuclear facility work for detecting radioactivities which might be released into the atmosphere by an accident of the nuclear facility. On the other hand, similar monitoring techniques are applied to an international monitoring network for CTBT (Comprehensive Nuclear-Test-Ban Treaty). The network is prepared for verification of compliance with this treaty, and for detecting a nuclear explosion. For this purpose, 80 atmospheric radionuclide monitoring stations will be set up all over the world, and among them two radionuclide stations will be established in Japan (Takasaki and Okinawa). An automatic monitoring system which is commercially available will be installed at each of Japanese stations. There are requirements to be met by the system such as the detection limit and the data availability, and these are severer than those for environmental monitoring systems in use. If the data obtained from the CTBT monitoring network are opened for use in research fields, they could be useful not only for the monitoring of radioactivities caused by a nuclear power accident but also in the fields of environmental sciences and earth sciences. In this report, performance of a CTBT monitoring system is compared with that of the environmental monitoring system in use, and applications of the CTBT monitoring data to the environmental and earth sciences are considered. (author)