PURPOSE: To enable easy production of 3-methylglutaric acid which is a main starting material for nylon from methyl-pentanediol or the like, a readily available substance, in an industrial scale by oxygen-oxidizing the substance in the presence of an oxidation catalyst and water.

CONSTITUTION: A starting compound selected from β-methyl-δ-valero-lactone, 2-hydroxy-4-methyltetrahydropyran and 3-methyl-1,5-pentanediol is oxidized with oxygen in the presence of an oxidation catalyst, for example, platinum and bismuth on activated carbon, and water, preferably at 80 to 150°C to obtain the compound. The amount of the water used is preferably 2 to 60 moles per mole of the starting compound. When the reaction is carried out in the presence of an acidic compound in an amount of 0.1 to 5wt.% based on the starting compound, the reaction rate is markedly improved and the acidic compound is preferably a solid acid such as an acidic ion-exchange resin or the like.