We investigate the vertex operators of the supergravity multiplet in IIB(IKKT) matrix model by calculating the disk amplitudes, exploiting the technique of conformal field theory. The vertex operators of IIB matrix model are given as the coupling of a closed string and the open strings which are introduced by the existence of D(−1)-branes. We consider the most generic couplings which involve both the bosonic and fermionic open strings. Our results are consistent with the previous results based on supersymmetry. We thus confirm the structure of the IIB matrix model vertex operators from the first principle.