"A Song of Humility (Gyeonggeunjigok)" compiled in The Annals of King Sejo is a musical piece based on the tunes of "A Special Song of Green Mountain (Cheongsan Byeolgok)", a Goryeo song by an unknown author. Consisting of nine stanzas (gok) each of which has twelve lines (haeng), "A Song of Humility" tried to generate new features by introducing several tones to the words of the song. The new tunes are: Daegang 1 (A), 4 and 5 (B) in the first line, Daegang 1 (C) in the third line, Daegang 4 in the fifth line, Daegang 5 (E) in the sixth line and Daegang 4 (F) in the seventh line. These six new tunes are achieved by using tonal sounds. In other words, when a word is of "average tone (pyeongseong)" it is expressed as "Low One (ha-il)" or Palace (gung). By the same token, a word of "high tone (sangseong)" goes up from "High Three (cheongtaejok)" or "High One (namgung)" to "High Two (cheonghwangjong)" whereas a word of "falling tone (geoseong)" or "entering tone (ipseong)" is expressed as "High One (namgung) or "High Two (cheonghwangjong)". However, the tone of "High Three" was only used for the first stanza while for the rest stanzas "High One" and "High Two" is used successively instead of "High Three" tones. Summing up, "A Song of Humility" was able to produce tunes that were different from those of "A Special Song of Green Mountain" by applying the tonal changes of Gasa to specific words in the work, and eventually created a new song that has gone far from the original Goryeo song. That the words where the tonal system was applied were all in Chinese characters means that the work is different from "Invitation of Peace (Chihwapyeong)", a musical piece composed under the reign of King Sejong, that had also used the system. At the same time, the application of the tonal system was less strict in this work compared with "Invitation of Peace", revealing that the use of the system was more flexible under the reign of King Sejo than in the days of King Sejong. Artists of the early Joseon Period tried to create a new musical work via giving some changes to an existing piece. As for "A Song of Humility", the means they chose was to use the tonal system for the words in the song. This study of "A Song of Humility" helps have better knowledge of the background of the works as well as the specific means used for the creation of musical pieces in the early Joseon period.