A dialogue system includes a processor configured to: generate a meaning representation corresponding to an input sentence by performing Natural Language Understanding on the input sentence, generate an output sentence corresponding to the input meaning representation based on Recurrent Neural network (RNN), and determine whether the input sentence cannot be processed using the natural language generator. The processor calculates a parameter representing a probability of outputting the input sentence when the meaning representation corresponding to the input sentence is input to the natural language generator, and determines whether the input sentence cannot be processed based on the calculated parameter.