12. The Hangeul input method of claim 11 , wherein the basic consonants assigned to the selected consonant key are sequentially and circularly selected by repeatedly operating the selected consonant key, the basic vowels assigned to the selected vowel key are sequentially and circularly selected by repeatedly operating the selected consonant key, and the extended consonants to be converted from the corresponding basic convertible consonants are sequentially and circularly selected by repeatedly operating the shift key.
12. The Hangeul input method of claim 11 , wherein the basic consonants assigned to the selected consonant key are sequentially and circularly selected by repeatedly operating the selected consonant key, the basic vowels assigned to the selected vowel key are sequentially and circularly selected by repeatedly operating the selected consonant key, and the extended consonants to be converted from the corresponding basic convertible consonants are sequentially and circularly selected by repeatedly operating the shift key. 15. The Hangeul input method of claim 12 , wherein the selected basic consonant, extended consonant, basic vowel, and combined vowel are displayed on a display screen, the display screen including: an input window for displaying the selected basic consonant, extended consonant, basic vowel, and combined vowel; and an input candidate group display window for sequentially displaying the basic consonants and basic vowels assigned respectively to the consonant keys and vowel keys when selecting the consonant keys and vowel keys, and displaying the extended consonants converted from the corresponding basic convertible consonants by the shift key in the sequence of an aspirate of the corresponding basic convertible consonants and a fortis of the corresponding basic convertible consonants when selecting the shift key.