Ultrafast optical response has been investigated using fs laser system on a perovskite-type cobalt oxide, Pr0.5Ca0.5CoO3 which undergoes the insulator-metal transition at 89 K. After the photo-irradiation at 30 K, the reflectivity shows a sudden increase. The ultrafast reflectance changes are quantitatively reproduced by the numerical calculation considering the linear combination of the dielectric functions of the initial insulating and the final metallic state, indicating ultrafast phase transition on the time scale of femtosecond.