Abstract: Provided is a method of operating a hydrolytic separator in which ammonia gas to be used as a reducing agent in a flue gas denitrization apparatus is generated by the hydrolysis of an aqueous urea solution. When the hydrolytic separator is started and ammonia gas injection is initiated, the hydrolytic separator is operated in a constant-pressure mode in which the internal pressure of the hydrolytic separator is kept constant regardless of the increasing temperature of the hydrolytic separator. Thereafter, the constant-pressure operation is switched to a variable pressure operation in which the pressure is raised as the temperature of the hydrolytic separator rises. In the method, the constant-pressure operation is switched to the variable pressure operation after the temperature of the hydrolytic separator in the constant-pressure operation has reached or exceeded the variable pressure operation temperature corresponding to that pressure.