Provided is a process for producing chlorosulfonyl isocyanate by reacting sulfur trioxide with cyanogen chloride, which comprises feeding sulfur trioxide and cyanogen chloride simultaneously to the reaction zone and maintaining the temperature of the reacting zone at 10 DEG to 50 DEG C. This is a commercially very simple process and can produce chlorosulfonyl isocyanate with high purity in high yield.
