Provided is a data processing system which includes: a first computer for receiving a processing request for a task processing, executing the processing, and holding data used therein; and a second computer for holding a duplicate of the data held in the first computer, halting the first computer if the first computer is determined to be halted, and receiving and processing the processing request. The first computer receives at least an update request as the processing request including request identification information to which unique numbers assigned to the individual processing requests in an ascending order are allocated, updates the held data, and transmits the update request including the request identification information to the second computer. The second computer stores a transmitted reference request and the update request as the processing requests, and processes the processing requests in an ascending order of the unique numbers included in the individual processing requests.