1. A response generation method comprising: a step of recognizing a voice of a user by performing voice recognition processing based on the voice of the user acquired though a microphone and generating character-string information of the voice of the user; a step of analyzing a structure of the recognized voice for the character-string information indicating recognized voice information of the user by using a structure analysis unit; and a step of generating a free response sentence in response to the voice of the user based on the analyzed voice structure by using a response generation unit and outputting the generated free response sentence by using a speaker, wherein the response generation method further comprises: a step of generating a repeat response sentence by extracting a keyword and its part of speech from the character-string information of the recognized voice of the user, selecting an additional word corresponding to the extracted keyword and its part of speech based on an additional information stored in a memory, and adding the selected word to the extracted keyword; a step of analyzing a phoneme of the voice of the user by detecting a change in a volume level or a frequency of the voice of the user acquired through the microphone to infer a break in the voice of the user; a step of generating a responsive response to the voice of the user based on an analysis result of the phoneme; and a step of outputting the generated repeat response sentence before outputting the generated free response sentence based on the voice structure by using the speaker, and the generated responsive response is output before the generated repeat response sentence is output.