The CKM angle phi3 is the least well known of the angles of the unitarity triangle and the only one that is accessible with tree-level
decays in a theoretically clean way. The key method to measure phi3 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. This talk discusses the results from Belle as well as the ongoing studies. This talk
will also explain the details of the planned measurement at Belle II, a substantial upgrade of the Belle detector and will operate at the
SuperKEKB energy-asymmetric $e^+ e^-$ collider.