A method of producing a radioactive-fluorine-labeled compound has a step of heating in a reaction vessel a mixture containing <18> F fluoride ions, a phase transfer catalyst, potassium ions, and water to evaporate water from the mixture (S10), and in the above step has a step of measuring a temperature of an outlet tube for discharging evaporated water during the heating of the reaction vessel (S12), and the evaporation process is finished at a timing determined based on the result of temperature measurement in the step (S12) of measuring the temperature (S16). Consequently, the amount of water present in the mixture can be controlled to an appropriate range and a stable yield can be achieved in the method of producing a radioactive-fluorine-labeled organic compound.