An exciton confined at a two-dimensional stacking fault in Bi13 crystal reveals large optical nonlinearity and ultra fast response for high density excitation. It shows three resonant luminescences whose decay-time constants are 20ps, 40ps and 900ps from the higher energy states reflecting cascade-type relaxation processes. By the degenerate four-wave mixing (DFWM) measurements, the dephasing times of three exciton states were obtained as 8.6ps, 15ps and 34ps. The dephasing at the lowest state is governed mainly by the LA-phonon intra-band scattering. While, at the higher states, it is governed by the cascade relaxation to the lower states in addition to the phonon scattering. It is found that the dephasing is also influenced by the exciton-exciton scattering under the high density excitation. A nonlinear transition involving a multi-photon process is observed in the excitation response of the DFWM signal. This result suggests the existance of the excitonic molecule.