In past 15 years, a significant progress is made in the field of hadron spectroscopy mainly driven by B-factory experiments. A large number of hadrons which can not be regarded as conventional meson or baryon, called exotic hadron, are discovered. These hadrons may shed light on the problems of low energy QCD such as mass generation or quark confinement.

Among many exotic hadrons, I will focus on the poster boy: X(3872). First, the review on the past studies of X(3872) will be presented. Then, recent measurement of the absolute branching fraction of B-> K+ X(3872) decay and its impact on the nature of X(3872) is presented. Finally, prospect toward understanding of X(3872) at Belle II experiment is given.