Abstract: A method of sensing an abnormal condition in a heat exchange process, capable of promptly sensing leakage of a process fluid in a process of heat exchange between a heat medium and a process fluid, is provided. With attention being paid in a process of heat exchange between a heat medium which is fused salt including nitrate and/or nitrite and a process fluid including secondary alcohols and/or a dehydrated product thereof to generation of a large amount of hydrogen gas resulting from contact between the leaked process fluid and the heat medium, the present invention is directed to a method of sensing an abnormal condition in a heat exchange process having a sensing step of sensing a hydrogen gas in a vapor phase portion in a flow path for the heat medium.