A continuous process for producing O,O-di-lower alkylchlorothiophosphate very important as an intermediate for organo-phosphate series agricultural chemicals which comprises continuously reacting phosphorus sulfochloride with a lower alcohol in the presence of caustic soda according to a multi-stage reaction form comprising from 3 to 8 stages in which the following conditions are satisfied:(1) The amounts of caustic soda and a lower alcohol used shall be from 2 to 2.2 moles and 10 moles or more, respectively, based on 1 mole of phosphorus sulfochloride.(2) Caustic soda, water and the lower alcohol shall be used as a caustic soda/water/lower alcohol solution, and besides the caustic soda concentration of the aqueous caustic soda in said solution shall be from 30.0 to 99.5 wt.%.(3) To the 1st reaction stage shall be continuously supplied the whole amount of phosphorus sulfochloride required and from 45 to 85 wt.% each of the amounts of caustic soda and the lower alcohol required; and to the 2nd and subsequent reaction stages shall be continuously supplied the residual caustic soda and lower alcohol in divided portions so that the substantial amounts supplied become successively small toward the final reaction stage.(4) The temperature of the reaction system shall be -5.degree. C. or lower.(5) The residence time in the reaction system shall be within 4 hours.