  We suggest that the boundary cosmological constant \zeta in c<1 unitary
string theory be regarded as the one-dimensional complex coordinate of the
target space on which the boundaries of world-sheets can live. From this
viewpoint we explicitly construct analogues of D-instantons which satisfy
Polchinski's ``combinatorics of boundaries.'' We further show that our operator
formalism developed in the preceding articles is powerful in evaluating
D-instanton effects, and also demonstrate for simple cases that these effects
exactly coincide with the stringy nonperturbative effects found in the exact
solutions of string equations.
