  The diquark has been considered to be important effective degrees of freedom
in hadron physics, especially for multi-quark physics and the structure of
heavy hadronic states. Using the Schwinger-Dyson formalism, we investigate the
non-perturbative effect of gluons for scalar diquarks with
renormalization-group improved coupling in the Landau gauge. Here, we treat the
scalar diquark as an effective degree of freedom with a peculiar size, while
the diquark is originally a bound-state-like object of two quarks. Since the
diquark has a non-zero color charge, it strongly interacts with gluons. We
evaluate the gluonic non-perturbative effect to the diquark, considering the
size effect of the diquark. We investigate the mass function of the diquark in
both cases with a constant bare diquark mass and twice of the running quark
self-energy. It is found that the diquark, especially the small diquark,
obtains a large effective mass by the gluonic dressing effect. The scalar
diquark mass seems to be dynamically generated by the non-perturbative effect,
although it does not have the chiral symmetry.
