Blood products for transfusion may undergo structural or chemical changes when mixed with other infusion fluids, potentially causing hemolysis or coagulation. Therefore, it is generally preferable to use a single infusion route. However, it is often difficult to secure multiple infusion routes in clinical settings. Case: A one-year-old infant was admitted to our hospital for infusions of fresh frozen plasma (FFP; 20 ml/h infusion) for the treatment of solid tumor. A string-like structure was observed in the infusion route after administration of FFP. The structure disappeared after the degradation test using plasmin, leading to the conclusion that it was composed of fibrin. Discussion: Reacting FFP with calcium results in fibrin precipitation. Given that Ringer's solution which contains calcium was used as the main infusion in this patient, we hypothesize that the saline washout of Ringer's solution after the prior infusion was insufficient at the time of FFP administration. It is also possible that a slow infusion rate and FFP stagnation in the infusion route contributed to fibrin deposition. Conclusion: This case emphasizes the importance of performing a thorough saline washout of the infusion fluid prior to the transfusion and preventing blood products from mixing with other infusion drugs.