This research was carried out to find how Chongmyo cheryeak 宗廟祭禮樂 (Royal ancestral shrine music) has been changed. All the available materials were analyzed and resulted in the following conclusions. First, the melody line of Chongmyo cheryeak has never been changed, but only the duration of every notes changed. The change of duration of each note resulted in the ruin of the rhythmic patterns that is one of the most important factors of Korean traditional music. Second, in the whole history of Chongmyo cheryeak no gradual change is found. The change happened only once or twice. Some music changed from the original Music Book of the Sejo sillok 『世祖實錄』right into one-tone one-bear style like the ones of present days. Others show another change between the original and those of present days. Third, the music book used by Yiwangjik aakpu 李王職雅樂部 musicians during the Japanese occupation was made with the fifth chapter (kwŏn 卷) of Sogak wŏnbo 『俗樂源譜』(lit. Source of Popular Music) based on Akchang yoram 『樂章要覽』(Text of Ritual Music) written in 1809. They put the music of Akchang yoram into square notation to make it look like music of one-tone one-beat style. It could be confirmed that there was no musical change in Chongmyo cheryeak until 1809 by the records of the King Sunjo's 純祖 (r.1801-1834) chronicles. So the music of fifth chapter of Sogak wŏnbo cannot be of one-tone one-bear style. All the music included in this Music Book was distorted by changing the duration of every note into one bear or two during the Japanese occupation. Fourth, the instrumentation of Chongmyo cheryeak was also distorted during the Japanese occupation. The biggest change was disappearance of the three stringed instruments (samhyŏn 三絃 : kŏmun'go 거문고, kayagŭm 伽倻琴, pip'a 琵琶) that had always been included in the instrumentation of tŭngga 登歌 (the terrace otchestra) and that all the Koreans have been very proud of.