A method of reducing a temperature difference between a high-temperature and a low-temperature substrate includes interposing a heat transfer facilitating layer which has a higher thermal conductivity than air and can hold particles between the substrates, and maintaining close contact between the high-temperature substrate, the heat transfer facilitating layer, and the low-temperature substrate, wherein formation of an air layer can be at least substantially prevented between the high-temperature substrate and the heat transfer facilitating layer, and between the low-temperature substrate and the heat transfer facilitating layer. A fluid reaction device includes a microfluidic reaction chip which accommodates a fluid, a heater, and a heat transfer facilitating layer which is interposed between the microfluidic reaction chip and the heater, the heat transfer facilitating layer has a higher thermal conductivity than air and can hold particles, wherein formation of an air layer can be prevented.