  The large-N reduced models have been proposed as the nonperturbative
formulation of the superstring theory. One of the most promising candidates is
the IIB matrix model. While there have been a lot of interesting discoveries of
the IIB matrix model in relation to the gravity, we have a lot of problems to
surmount, if a large-N reduced model is to be an eligible framework to unify
the gravitational interaction. Firstly, it is still an enigma how we can
realize the local Lorentz invariant matrix model. In addition, we need to
understand how we can describe the curved spacetime more manifestly, in terms
of a large-N reduced model.
  This thesis discusses several attempts to address these issues concerning the
gravitational interaction. This thesis is based on the following author's works
hep-th/0102168, hep-th/0204078, hep-th/0209057 and hep-th/0401038.
