How many movies have Hideki Takahashi and Masato Kai collaborated on?

They have worked on 4 movies together! Their first movie together was Kyotaro Nishimura's Travel Mystery - Soya Honsen Murder Incident (2001) and was their highest rated movie together. They have not worked together since Kyotaro Nishimura Travel Mystery 68: Disappearing Woman in Yamagata (2017). They are most known for their mystery films together.