8. A method for encoding binary data into NRZI codes comprising the steps of: converting binary data words each containing N data bits (wherein N is an integer larger than 1) into binary channel words each containing M data bits (wherein M is an integer larger than N); and NRZI modulating said channel words into NRZI coded channel codes, wherein said converting step comprises the steps of: holding a specific data word which satisfies at least one of a condition that the specific data word has a specific bit pattern and a condition that the specific data word has a specific relationship to adjacent data words; converting said held specific data word to an established channel word so that absolute value of digital sum variation of previously encoded binary data is reduced when another specific data word which satisfies at least one of said conditions has been inputted; and outputting said established channel word after at least a delay time required to convert said held specific data word to said established channel word, and wherein, if a predetermined number of data words which are convertible to channel words in real time have been inputted before said another specific data word is inputted, said step of converting said held specific data converts said held specific data to said established channel word so that absolute value of digital sum variation of previously encoded binary data is reduced when said predetermined number of data words have been inputted.
8. A method for encoding binary data into NRZI codes comprising the steps of: converting binary data words each containing N data bits (wherein N is an integer larger than 1) into binary channel words each containing M data bits (wherein M is an integer larger than N); and NRZI modulating said channel words into NRZI coded channel codes, wherein said converting step comprises the steps of: holding a specific data word which satisfies at least one of a condition that the specific data word has a specific bit pattern and a condition that the specific data word has a specific relationship to adjacent data words; converting said held specific data word to an established channel word so that absolute value of digital sum variation of previously encoded binary data is reduced when another specific data word which satisfies at least one of said conditions has been inputted; and outputting said established channel word after at least a delay time required to convert said held specific data word to said established channel word, and wherein, if a predetermined number of data words which are convertible to channel words in real time have been inputted before said another specific data word is inputted, said step of converting said held specific data converts said held specific data to said established channel word so that absolute value of digital sum variation of previously encoded binary data is reduced when said predetermined number of data words have been inputted. 10. A method according to claim 8, wherein said step for converting said data words to said channel words is performed according to the following rules: (i) two or more `0` bits exist between two `1` bits in said channel word if said channel word contain two or more `1` bits; (ii) a most significant bit or least significant bit of each channel word is made normally `0` as a merging bit, and when bits adajcent to said merging bit at both sides thereof are both `1`, said adjacent bits are both made `0` and said merging bit is made `1`; and (iii) said merging bit is made `0` or `1` so that the absolute of DSV is reduced when a bit which is three bits before said merging bit is `1` and said merging bit is followed by two continuous `0` bits or when two continuous bits just before said merging bit are both `0` and a bit three bits after said merging bit is `1`.