A method for producing chlorosulfonyl isocyanate by reaction of sulfur trioxide with cyanogen chloride, wherein chlorosulfonyl isocyanate or a solution including chlorosulfonyl isocyanate is used as a reaction solvent, sulfur trioxide and cyanogen chloride which are respectively diluted with the chlorosulfonyl isocyanate or the solution including chlorosulfonyl isocyanate are added at the same time to a reaction system in an almost equimolar amount under reflux. By the production method of present invention, chlorosulfonyl isocyanate can be produced from sulfur trioxide and cyanogen chloride in which the yield of the chlorosulfonyl isocyanate is high, the method has excellent operability, number of equipments is reduced, and time for controlling the temperature is saved.