1. A text paraphrasing method for generating, with respect to a question text inputted by a user, a question text of different expression but the same content, said method comprising: extracting a combination of sentences having a degree of similarity greater than or equal to a set threshold, from among question texts having the same answer texts stored in storage unit, termed “first sentence extraction step”; computing a conversion rule for converting from said combination of sentences extracted in said first sentence extraction step, to a sentence of the same content but different expression, termed “conversion rule computation step”; extracting again from said storage unit a combination of sentences having a degree of similarity greater than or equal to said threshold, using said conversion rule computed in said conversion rule computation step, termed “second sentence extraction step”; judging whether or not all of said combinations of sentences extracted in said first and said second sentence extraction steps match, termed “judging step”; and generating a paraphrase text of said inputted question text, using said conversion rule computed in said conversion rule computation step, termed “text generation step”; wherein execution of said first sentence extraction step, said conversion rule computation step, and said second sentence extraction step is repeated until it is judged that said combinations of sentences extracted in said first and said second sentence extraction steps all match.