Abstract: A heat medium contains liquefied isobutane, liquefied carbon dioxide, and liquid nitrogen. The content of liquefied isobutane is 20 to 30% by mass of the total amount of the heat medium, the content of liquefied carbon dioxide is 50 to 70% by mass of the total amount of the heat medium, and the content of liquid nitrogen is 10 to 20% by mass of the total amount of the heat medium. Even if containing flammable liquefied isobutane, the heat medium is non-flammable because it is mixed with non-flammable liquefied carbon dioxide. The heat medium does not contain chlorine or fluorine, so that it has a low environmental load.