A design support system comprising: a designer terminal for use by designers; a maintenance personnel terminal for use by maintenance personnel; and a design support device capable of communicating with the designer terminal and the maintenance personnel terminal, wherein the design support device includes a failure causal model database configured to store a failure causal model of a target equipment, a failure causal model updating unit configured to update the failure causal model based on an assumed cause of a failure of the target equipment input from the designer terminal, a maintenance support unit configured to estimate a type and a cause of a failure having a high occurrence probability based on a symptom of the target equipment corresponding to a symptom of a failed target equipment input from the maintenance personnel terminal when the failed target equipment is maintained and based on the failure causal model updated by the failure causal model updating unit, and configured to output the estimated result to the maintenance personnel terminal, wherein the design support system is characterized by comprising: a maintenance record database configured to store, as a maintenance record, authenticity information for the type of the failure and the cause thereof estimated by the maintenance support unit, wherein the authenticity information is input from the maintenance personnel terminal based on the type of the failure and the cause thereof found as a result of an actual maintenance work, and a failure cause analysis unit configured to update the failure causal model of the failure causal model database based on the authenticity information stored as the maintenance record in the maintenance record database, configured to estimate a failure cause of the target equipment based on the failure causal model updated by the failure cause analysis unit, and configured to output the estimated failure cause of the target equipment to the designer terminal. a designer terminal for use by designers; a maintenance personnel terminal for use by maintenance personnel; and a design support device capable of communicating with the designer terminal and the maintenance personnel terminal, wherein the design support device includes a failure causal model database configured to store a failure causal model of a target equipment, a failure causal model updating unit configured to update the failure causal model based on an assumed cause of a failure of the target equipment input from the designer terminal, a maintenance support unit configured to estimate a type and a cause of a failure having a high occurrence probability based on a symptom of the target equipment corresponding to a symptom of a failed target equipment input from the maintenance personnel terminal when the failed target equipment is maintained and based on the failure causal model updated by the failure causal model updating unit, and configured to output the estimated result to the maintenance personnel terminal, wherein the design support system is characterized by comprising: a maintenance record database configured to store, as a maintenance record, authenticity information for the type of the failure and the cause thereof estimated by the maintenance support unit, wherein the authenticity information is input from the maintenance personnel terminal based on the type of the failure and the cause thereof found as a result of an actual maintenance work, and a failure cause analysis unit configured to update the failure causal model of the failure causal model database based on the authenticity information stored as the maintenance record in the maintenance record database, configured to estimate a failure cause of the target equipment based on the failure causal model updated by the failure cause analysis unit, and configured to output the estimated failure cause of the target equipment to the designer terminal.