To provide a production process for benzene nucleus chlorinated 2-formylbenzoic acid that can product 4-COFBA and 5-COFBA from a commercially readily available raw material with industrial advantage.

In this process, the synthetic intermediate of acid chloride mixture, for example, an acid chloride mixture of 4-chloro-2-dichloromethylbenzoyl chloride and 5-chloro-2-dichloromethylbenzoyl chloride that is prepared on the way of the process are used without isolation and purification to produce a mixture of formylbenzoic acids including 4-chloro-2-formylbenzoic acid and 5-chloro-2-formylbenzoic acid. This formylbenzoic acid mixture is crystallized out of water to separate into 4-chloro-2-formylbenzoic acid and 5-chloro-2- formylbenzoic acid thereby affording these 4-chloro-2-formyl-benzoic acid and 5-chloro-2-formylbenzoic acid.