Nano-particles 1 of a metal or the like are dispersed in a liquid fluid 2. Thereby, the reactivity or toxicity of the liquid fluid can be reduced when the liquid fluid has reactivity or toxicity. The flow resistance of the liquid fluid can be raised, and the leakage of the liquid fluid from minute cracks can be reduced. By using the liquid fluid as a heat transfer medium of a heat exchanger, the heat transfer performance equivalent to or higher than the heat transfer performance of the original heat exchanger can be obtained.