The song is called "Sorrow".

"Sorrow" is pure music without lyrics. It was composed by Takanashi Koji, arranged by Takanashi Koji, and performed by 634. The song is included in the album "NARUTO-NARUTO-オリジナルサウンドトラックIII", which was released on April 23, 2005. The whole song is foreshadowed by violin and guitar; the desolate and sad voice of Shakuhachi is accompanied by the slow and gentle sound of piano.

Extended information:

Other versions of the song "Sorrow":

1. The Oturans version of "Sorrow" is pure music without lyrics, composed by A song played by Oturans. This song is included in the album "NARUTO" and was released on October 7, 2017.

2. Bai's version of "Sorrow" is pure music without lyrics, a song performed by Bai. The song is included in the album "Sorrow" and was released on July 12, 2018.

3. Ke Youdao's version of "Sorrow" is pure music without lyrics, a song performed by Ke Youdao. This song is included in the album "NARUTO Mix" and was released on April 25, 2020.