In recent years, exotic hadrons called X, Y, Z which cannot be explained by the quark model have been found one
after another. The tetra-quark state, which is one of the typical scenarios to interpret the exotic state, is based on
the existence of a diquark state. The discovery of a pentaquark state at CERN in 2015
also makes a diquark state even more important. Therefore, it is considered that a diquark state plays
an important role in many kinds of QCD physics.
We discuss existence of such diquarks from the viewpoint of a first principle calculation.