We report the picosecond time resolved spectra of the optical Stark effect on a quasi-two-dimensional exciton localized at a stacking fault plane in Bil3crystals. The ultrafast response of the blue-shift due to the optical Stark effect(OSE) is confirmed in picosecond time domain under negative detuning excitation condition. For positive detuning excitation, an ultrafast switching from the red shift by the OSE to the blue-shift due to the nonlinear interaction with the large amount of incoherent “real” exciton occurs. Under the just resonance excitation, the blue-shift due to the “real” exciton is also observed during several tens of picoseconds after the pumping, and an absorption bleaching occurs at the timing of the pump-pulse peak. The bleaching is discussed in connection with the OSE.