  This is a thesis for Rigaku-Hakushi($\simeq$ Ph. D.). It clarifies the
geometric meaning and field theoretical consequences of the spectral flows
acting on the space of states of the `$G/H$ coset model'. As suggested by Moore
and Seiberg, the spectral flow is realized as the response of states to certain
change of background gauge field together with the gauge transformation on a
circle. Applied to the boundary circle of a disc with field insertion, such a
realization leads to a certain relation among correlators of the gauged WZW
model for various principal $H$-bundles. In the course of derivation, we find
an expression of a (dressed) gauge invariant field as an integral over the flag
manifold of $H$ and an expression of a correlator as an integral over a certain
moduli space of holomorphic $H_{\bf
  C}$-bundles with quasi-flag structure at the insertion point. We also find
that the gauge transformation on the circle corresponding to the spectral flow
determines a bijection of the set of isomorphism classes of holomorphic $H_{\bf
C}$-bundles with quasi-flag structure of one topological type to that of
another. As an application, it is pointed out that problems arising from the
field identification fixed points may be resolved by taking into account of all
principal $H$-bundles.
