Provided is a process for producing 3,5-dimethyldodecanoic acid, which is an active ingredient of the pheromone of California prionus. More specifically provided is a method for producing 3,5-dimethyldodecanoic acid (5) comprising the steps of converting 2-methylnonyl halide (1) to a 2-methylnonyl metal reagent (2), and reacting the 2-methylnonyl metal reagent (2) with 2-ethylidene malonic acid ester (3) to form 2-(1,3-dimethyldecyl)malonic acid ester (4) as a result of 1,4-addition of the 2-methylnonyl metal reagent (2) to the 2-ethylidene malonic acid ester (3), as shown in the following scheme: