Author Eiichiro Oda's pirate adventure manga One Piece manages to prove that it's the most popular manga in Japan. Oricon recently announced that the manga ranks 1st in the "top selling manga in Japan", selling 18,151,599 copies in the past year.

Furthermore, One Piece Volume 69 ranked 1st in the "top selling manga volume in Japan" for 2013, with volumes 70 to 72 at the second to fourth spot. Meanwhile, six volumes of Hajime Isayama's survival manga Attack on Titan conquered the fifth through tenth spot.

Top 10 Top-Selling Manga in Japan by Volume: 2013
1. One Piece 69 - 3,147,224 - Eiichiro Oda
2. One Piece 70 - 3,039,487
3. One Piece 71 - 2,890,263
4. One Piece 72 - 2,354,005
5. Attack on Titan 9 - 1,950,807 - Hajime Isayama
6. Attack on Titan 10 - 1,915,976
7. Attack on Titan 11 - 1,453,879
8. Attack on Titan 8 - 1,339,845
9. Attack on Titan 1 - 1,299,048
10. Attack on Titan 2 - 1,267,129

Top 10 Top-Selling Manga in Japan by Series: 2013
1. One Piece - 18,151,599 - Eiichiro Oda
2. Attack on Titan - 15,933,801 - Hajime Isayama
3. Kuroko's Basketball - 8,761,081 - Tadatoshi Fujimaki
4. Magi - 7,151,001 - Shinobu Ohtaka
5. Naruto - 5,553,933 - Masashi Kishimoto
6. Silver Spoon - 4,858,699 - Hiromu Arakawa
7. Assassination Classroom - 4,595,820 - Yusei Matsui
8. Hunter × Hunter - 4,231,475 - Yoshihiro Togashi
9. Fairy Tail - 3,790,151 - Hiro Mashima
10. Terra Formars - 3,602,630 - Yū Sasuga/Kenichi Tachibana

This list covers sales surveyed from November 19th, 2012 to November 17th, 2013.

Source: Oricon via ANN (link 2)