PURPOSE: To provide the method for producing the naphthoic acid and the naphthaldehyde, capable of reducing the load of a rectification device and capable of separately obtaining the naphthoic acid and the naphthaldehyde in high purities and in high yields.

CONSTITUTION: In the method for producing the naphthoic acid and the naphthaldehyde by oxidizing methylnaphthalene with a molecular oxygen- containing gas in the presence of an oxidation catalyst in a solvent containing an aliphatic monocarboxylic acid, the improvement comprises adding water to the oxidation reaction solution to separate the naphthoic acid and the naphthaldehyde from the liquid phase, as such or after the solvent is distilled out to concentrate the solution, dissolving the naphthoic acid in a basic aqueous solution, neutralizing the dissolution solution to separate and recover the naphthoic acid, and subsequently distilling the separation residues under vacuum to recover the naphthaldehyde.