An automatic circulation collection type data system is constructed by connecting a monitoring device and data collecting modules disposed at respective places through optical cables. The monitoring device transmits a trigger pulse for data collection through the optical cable to each data collecting module. An identification code is given to each data collecting module, and each data collecting module has an automatic circulation mechanism comprising first and second optical switches, a data collecting unit, and a controller 6 for performing various kinds of control for data collection. When receiving the trigger pulse from the monitoring device, the automatic circulation mechanism turns on the third switch and transmits the collected data through the optical cable to the monitoring device together with the identification code, and also transmits a trigger pulse through the optical cable to a next data collecting module.