  We show that the D-instanton in the $AdS_5 \times S^5$ background is a
wormhole connecting the background $AdS_5 \times S^5$ to the flat space $\bf
R^{10}$ located at the position of the D-instanton. By a $SL(2,{\bf R})$
rotation of type IIB theory, we can make the {\it global} geometry flat in
string frame. We also find that, due to the tight relation between the dilaton
and the axion, there is no $SL(2,{\bf R})$ element that takes strong string
coupling to weak one without making the axion ill defined. We also discuss the
case of $AdS_3$ as well as the instanton gases. A subtlety on the D-instanton
at the boundary or at the horizon is discussed.
