Data transfer monitoring system, data transfer monitoring method, and site system

This invention provides a technique for enabling data storage amounts and data transfer amounts to be suppressed, while enabling desired data used for monitoring to be transferred. A site system detects an alert; generates an index for determining collection target data that are data during a predetermined collection target time period including a time at which the alert occurred; causes the collection target data and the index to be recorded into a time sequence data table in correspondence with each other; and also causes alert identification information and the index to be stored into an index determination table in correspondence with each other. A center system receives the alert identification information and transmits, to the site system, a data request for requesting the collection target data. The site system, when having received the data request, searches the index determination table, thereby acquiring the index in correspondence with the designated alert identification information, then acquires, from the time sequence data table, the collection target data determined by use of the index, and then transmits the acquired collection target data to the center system.