A method of chemically extending a carbon chain of an unsaturated fatty acid for conversion into a different unsaturated fatty acid has been reported. The present invention shortens reaction steps of conventional methods, and completes a carbon chain extending reaction in a shorter time. The present invention provides a method of extending a carbon chain of an unsaturated fatty acid by two carbons, comprising steps of four stages including a short-path conversion reaction of an unsaturated fatty chain obtained from an unsaturated fatty acid into a malonic ester derivative, and heating of the malonic ester derivative to reflux in a lower fatty acid solution. The method of the present invention can complete a carbon chain extending reaction in a shorter time.