  We report on progress in determining the complete form of vertex operators
for the IIB matrix model. The exact expressions are obtained for those emitting
massless IIB supergravity fields up to sixth order in the light-cone
superfield, in which the conjugate gravitino and conjugate two-form vertex
operators are newly determined. We also provide a consistency check by
computing the kinematical factor of a four-point graviton amplitude in a
D-instanton background. We conjecture that the low-energy effective action of
the IIB matrix model at large N is given by tree-level supergravity coupled to
the vertex operators.
