  We review the recently proposed string theory in two dimensional black hole
background. Especially, the structure of the duality in the target space is
discussed. The duality is analogous to \lq\lq $R \rightarrow 1/R$" symmetry of
a compactified boson. We consider the duality in more general target space
manifolds which have Killing symmetries and we give an explicit formula which
connects two different manifolds which are dual to each other. Superstring
theory in two dimensional black hole background is also discussed based on
supersymmetric $SU(1,1)/U(1)$ gauged Wess-Zumino-Witten model.
