Provided is a process for producing 3,5-dimethyldodecanoic acid, which is an active ingredient of the pheromone of California prionus, and 4-carboxy-3,5-dimethyl-3,5-dodecadienoic acid which is an intermediate useful for producing 3,5-dimethyldodecanoic acid. More specifically, provided is a method for producing 3,5-dimethyldodecanoic acid, comprising the steps of subjecting 3-methyl-2-pentene-1,5-diacid diester (1) and 2-nonanone (2) to a condensation reaction and subsequent hydrolysis into 4-carboxy-3,5-dimethyl-3,5-dodecadienoic acid (3); decarboxylating or decarboxylating and hydrolyzing the 4-carboxy-3,5-dimethyl-3,5-dodecadienoic acid (3) into 3,5-dimethyl-2,4-dodecadienoic acid (4); and hydrogenating the 3,5-dimethyl-2,4-dodecadienoic acid (4) into 3,5-dimethyldodecanoic acid (5), as shown in the following scheme: