A word is input, along with a concept notation function that is a function for expressing what are described by that word, from a word set output unit to an optimal word string output unit. The optimal word string output unit computes a similarity between a word from the word set output unit and a registered word registered in a dictionary. The optimal word string output unit computes, even when a word not registered in the dictionary is input, a similarity between that word and a word registered in the dictionary.