The present invention relates to a method for the chemical conversion of an unsaturated fatty acid, particularly a carbon chain extension reaction. According to the present invention, a method for extending the length of a carbon chain in an unsaturated fatty acid by two carbon atoms is provided, said method comprising a step of heating a malonic acid ester derivative of an unsaturated fatty acid to reflux in a lower fatty acid solution in the presence of an antioxidative agent. It is preferred that the unsaturated fatty acid is an unsaturated fatty acid having 16 to 24 carbon atoms. It is preferred that the unsaturated fatty acid is selected from the group consisting of linoleic acid, linolenic acid, arachidonic acid, stearidonic acid, icosatetraenoic acid, icosapentaenoic acid, tetracosahexaenoic acid and docosahexaenoic acid. According to the method of the present invention, a carbon chain extension reaction can be completed within a shorter time.