A reactor for producing a nitrile compound from a carbon ring or heterocyclic compound having organic substituents by a gas phase reaction using a fluidized catalyst bed with ammonia and a gas containing oxygen. In a cylindrical fluidized catalyst bed having a diameter of 2.0 meters or greater, partial vaporization-type cooling tubes (the cooling medium is partially vaporized in the tubes) and complete vaporization-type cooling tubes (the cooling medium is completely vaporized in the cooling tubes) are disposed in a specific arrangement. Water containing ionic SiO2 in 0.1 ppm or smaller and having an electric conductivity of 5 μS/cm or smaller is used as the cooling medium for the complete vaporization-type cooling tubes. The temperature of the reaction is easily stabilized and uniform distribution of temperature is obtained in the fluidized catalyst bed. Stable continuous operation is achieved for a long time in a commercial scale apparatus.