Errors in input data are concealed by interpolation using a mean value and the previous word which are automatically interchanged in accordance with the conditions of occurrence of errors. A series of data words in the input data is first modified into a form in which when the input word is erroneous it is replaced by correct input word immediately preceding the erroneous word. Then, in association with each word in the modified data word series a mean value between words immediately before and after that word or between that word and a word thereafter is produced, and if that word is the replaced word it is further replaced by the mean value produced in association with that data. Thus, an independent error is concealed by a mean value between the correct word immediately before and after the erroneous word while continuous errors are concealed in such a manner that the last erroneous word is replaced by the mean value between correct words occurring immediately before and after the continuous erroneous words and the other erroneous words are all replaced by the correct word occurring immediately before the continuous erroneous words.