  The CKM angle $\phi_3$ is less precisely known than the angle $\phi_1$ and
the only one that is accessible with tree-level decays in a theoretically clean
way. The key method to measure $\phi_3$ is through the interference between
$B^+\to D^0 K^+$ and $B^+ \to \bar D^0 K^+$ decays which occurs if the final
state of the charm-meson decay is accessible to both the $D^0$ and $\bar D^0$
mesons. To achieve the best sensitivity, a large variety of $D$ and $B$ decay
modes is required, which is possible at Belle experiment as almost any final
state can be reconstructed including those with photons. The results from
Belle, as well as the ongoing studies, are discussed here. The details of the
planned measurement at Belle II, a substantial upgrade of the Belle detector
which will operate at the SuperKEKB energy-asymmetric $e^+ e^-$ collider, are
also discussed. The results from the first collisions at Belle II during
April-July 2018 are shown too.
