It is an object to provide a data collection system that is configured to reduce a communication amount, etc. at the time when data are collected from a plurality of devices, so as to reduce a communication amount attended by the collection of data without increasing processing loads imposed on devices. A symbol classifying unit of a data relay device classifies received data that have been already compressed. A data recompressing unit replaces codes contained in the classified already compressed data with other codes, so as to recompress the already compressed data. A symbol set clustering unit sends a transfer destination renewal device a communication speed at the time when the recompressed data are transferred to other devices, a processing speed at the recompressing time, etc. The transfer destination renewal device generates transfer destination information on the basis of the communication speed, the processing speed, etc.