1. A voice interactive system comprising: a voice information input part for inputting user's voice information from a user terminal; a voice recognition part for conducting voice recognition with respect to the voice information, and analyzing contents of the voice information; a voice information mediation part for controlling a transmission path of the voice information in accordance with the contents of the voice information; an interaction engine for extracting contents of a response corresponding to the voice information by referring to a knowledge database, and creating a synthesized voice in accordance with the contents of a response; and a voice information output part for outputting the synthesized voice, wherein the voice information mediation part monitors at all times whether or not the user's interaction is being smoothly conducted, and in a case of determining that the user's interaction is not being smoothly conducted, allows a third-party user to participate in interaction between the user and the interaction engine from another terminal as a helper.