PURPOSE: To convert copper oxalate or an alkaline earth metal oxalate insolubilized and deposited in a reaction solution system into its alkali metal oxalate and to efficiently separate the alkali metal oxalate without using a special apparatus in a process for producing a carbonic acid ester without using phosgene.

CONSTITUTION: Alcohol is reacted with carbon monoxide and oxygen in the presence of a catalyst composed of copper and/or an alkaline earth metal to obtain a carbonic acid ester. In this process, an alkaline metal hydroxide is reacted with copper oxalate or an alkaline earth metal oxalate respectively insolubilized and deposited in the reaction solution to convert the oxalic acid residue as the by-product into its alkali metal oxalate and to remove it out of the reactional system. The alkali metal oxalate generated in this method is soluble in a solvent such as water or alcohol. On the other hand, by-products such as copper oxide copper hydroxide, an alkali hydroxide and an alkaline earth hydroxide are sparlingly soluble in the same solvent, and therefore, solidliquid separation can readily be carried out. By this method, oxalic acid residue can be efficiently removed and the metal component in the catalyst can be recovered. This invention is also an effective method for producing an alkali metal oxalate.