1. A natural language question answering system, comprising: one or more processors that process computer executable program code embodied in computer readable storage media, the computer executable program code comprising: conversion program code that generates a plurality of modified questions by paraphrasing a user's question; question answering engine program code that receives each of the user's question and the modified questions, and select candidate answers corresponding to each of the user's question and the modified questions; and detection program code that detects at least one among the searched candidate answers as an answer, wherein the conversion program code comprises: evaluation program code that confirms at least one among meaning preservation, grammar preservation, and an expression popularity on a plurality of paraphrase questions paraphrasing the user's question, and calculates a reliability value of the plurality of paraphrase questions; and prioritization program code that selects N modified questions in a sequence in which the reliability value is great among the plurality of paraphrase questions, and outputs the N modified question, and wherein the conversion program code calculates the reliability value of the plurality of paraphrase questions by considering a weight value with respect to a possibility in which a specific postposition or affix is added to a word or a phrase included in the plurality of paraphrase questions from a language model which is previously learned.