To provide an expression conversion device in compact constitution capable of performing a processing at a high speed and outputting a result for correctly telling an intention even when a recognition error is generated.

This device is provided with a dependency relation analysis means 2 for selecting important words from sentences for the respective sentences of a corpus 1, specifying the combination of the important words with co-occurrence relation from the prescribed number of the combinations of the important words and making the specified combination of the important words and an expression after the conversion of the sentence which is the object of selecting the important word correspond beforehand and an important word extraction means 5 and an example sentence selection means 7 for selecting the important words decided beforehand from an inputted sentence, combining them, comparing the combination of the combined important words with the combination of the important words specified beforehand of the respective sentences, selecting one, two or more sentences whose compared results match or are highly similar and outputting the expression after the conversion of the sentence.