Abstract: The process of the present invention is characterized in that when a carbonic acid diester is produced by catalytically reacting carbon monoxide with a nitrous acid ester in the presence of a solid catalyst, a small amount of a chloroformic acid ester is mixed into a material gas containing the carbon monoxide and the nitrous acid ester and thereby the catalyst employed can be maintained at a high activity condition over a long time period, and the carbonic acid diester can be produced stably over a long time period at a high reaction rate and with a high selectivity.