  The partition function of $D_{N+1}$ topological string, the coupled system of
topological gravity and $D_{N+1}$ topological minimal matter , is proposed in
the framework of KP hierarchy. It is specified by the elements of $GL(\infty)$
which constitute the deformed family from the $A_{2N-1}$ topological string.
Its dispersionless limit is investigated from the view of both dispersionless
KP hierarchy and singularity theory. In particular the free energy restricted
on the small phase space coincides with that for the topological
Landau-Ginzburg model of type $D_{N+1}$.
