A data processing apparatus has a dictionary control unit in which pieces of dictionary data stored in a dictionary buffer are compared with pieces of input data to be compressed input one after another, a piece of particular dictionary data agreeing with a piece of particular input data is expelled from the dictionary buffer, the dictionary data stored in the dictionary buffer are placed close together in a direction, and the particular input data is written in the dictionary buffer. Also, a data processing apparatus has an occurrence frequency producing editor for counting an occurrence frequency of each of types of pieces of input data to be compressed and producing an occurrence frequency table formed of a plurality of occurrence frequencies of the types of the input data to be compressed, a data comparing editor for predicting an occurrence of a piece of next input data subsequent to a piece of input data, and a code converting editor for converting the types of pieces of input data according to the occurrence prediction of the next input data in the data comparing editor on condition that a type of a piece of input data is converted into a piece of compressed data having a shorter bit length as an occurrence probability of the type of the input data is higher.