To carry out a kind of filtration for a plurality of word string candidates outputted from a recognition device by inferring a proper word string.

As a result of voice recognition of a user's speech, a plurality of word string candidates are produced, and a control device 32 carrys out a kind of filtration for the word string candidates. For example, selection and weighting on previously prepared sentence structure evaluation tables 34 are conducted (M5) from viewpoints such as grammar, colloquial expressions, and use frequency, or from viewpoints such as matching with circumstances, situations, and requirements of the user, and the degree of properness of a sentence structure is evaluated (M9) based on distribution of evaluation values. This enables a statistically proper word string (it can be said that this is proper in meaning) to be inferred (extracted). A tolerance of the whole word string is calculated (M8) from tolerances of respective words produced together with the word string and it can be used in the final evaluation (M9). In the case of a dialog system, context information in a dialog can be utilized as well.