Optical imaging of an exposed cortex for brain function measurement is an attractive method for both clinical and physiological investigations. Multi-spectral imaging of the exposed cortical tissue enables measuring the activity-dependent changes in oxy- and deoxy haemoglobin independently. Because light propagation in the cortical tissue strongly depends upon wavelength, the blurring by a scattering effect on multi-spectral images depends upon wavelength as well. It is important for more accurate measurement to correct this wavelength-dependent blurring in the multi-spectral images of the exposed cortex. In this study, the relative point spread functions which represent the difference in blurring by wavelength were predicted from the multi-spectral images of a blood vessel in the cortical tissue. The multi-spectral images of the cortical model are calculated by Monte Carlo simulation and wavelength-dependent point spread functions are estimated from the cross section of the blood vessel in the images. The tendency of the wavelength-dependence of relative point spread functions is almost the same as that of the point spread functions predicted from the light propagation in the cortical model. The relative point spread functions estimated from wide blood vessels are broader than those estimated from a narrow blood vessel.