1. A data conversion apparatus for converting first data into second data, comprising: class-tap generating means for generating, from the first data, a class tap used for classifying a specified item of the second data into a class of at least one class; classification means for classifying the specified item of the second data based on the class tap; predictive-tap generating means for generating a predictive tap for determining the specified item of the second data from the first data; tap-coefficient obtaining means for obtaining a tap coefficient for the class of the specified item of the second data from tap coefficients obtained by using second learning data corresponding to the second data and first learning data corresponding to the first data and by learning a relationship between the second learning data and the first learning data for each of said at least one class while applying weighting to the second learning data and the first learning data based on an activity of the predictive tap generated from the first learning data; and computation means for determining the specified item of the second data by using the predictive tap and the tap coefficient for the class of the specified item of the second data.