In the production of carboxylic acid or carboxylic acid ester, a reaction product containing the carboxylic acid or carboxylic acid ester which has been produced by the reaction of an olefin, carbon monoxide, and water or an alcohol in the presence of HF is heat-treated in the presence of an acid catalyst and an acid adsorbent. By the heat treatment, the contamination of the carboxylic acid or carboxylic acid ester with HF and/or fluorine compounds such as acyl fluorides can be prevented, to enable the stable production of a high quality and high purity carboxylic acid or carboxylic acid ester without causing troubles such as corrosion of apparatus.