  We discuss the open string ending on D $p$-branes in IKKT framework. First we
determine the boundary conditions of Green-Schwarz superstring which are
consistent with supersymmetry and $\kappa$-symmetry. We point out some
subtleties arising from taking the Schild gauge and show that in this gauge the
system incorporates the limited dimensional D $p$-branes ($p=3,7$). The matrix
regularization for the Dirichlet open string is given by gauge group SO(N).
When $p=3$, the matrix model becomes the dimensional reduction of a 6
dimensional ${\cal N}=1$ super Yang-Mills theory.
