6. The apparatus of claim 2 wherein output of the selecting means consists of the output from the first replacement means when there are only two code words W2a and W2a' corresponding to the inputted data word.
2. A code coversion apparatus for generating run length limited codes wherein m-bit data words are converted into n-bit code words which are concatenated to form a bit sequence where a number of successive bits having a same binary value is limited to a range of a smaller value d to a larger value k, the apparatus comprising: (a) first generation means for generating an n-bit code word W2a corresponding to an inputted m-bit data word; (b) second generation mean for generating an n-bit code word W2b corresponding to the inputted m-bit data word; (c) first replacement means for replacing the code word W2a by a code word W2a' in accordance with information related to a code word W1 preceding the code word W2 and with information related to the code word W2a, wherein the code word W2a' is precombined with the code word W2a; (d) second replacement means for replacing the code word W2b by a code word W2b' precombined with the code word W2b; (e) first delay means for delaying the code word W2a or 22a'; (f) second delay means for delaying the code word W2b or W2b'; and (g) means for selecting one of code words outputted from the first and second delay means in accordance with information related to the code word outputted from the first delay mean and with information related to the code word W2a outputted from the first generation means.