A new phenomenon of periodic oscillations is observed in I-V characteristics of porous silicon under illumination by visible light. The measurements are performed at room temperature using a scanning tunnelling microscope. The heights of the oscillation peaks appear to be a linear function of the oscillation number. The experimental value of the Coulomb energy determined from the oscillation period is much smaller than . The oscillations are attributed to a Coulomb effect, i.e. to the periodic trapping of a multi-electron level in a quantum well within a Si nanocrystal under the combined influence of the Coulomb interaction among the carriers and the variation of the potential difference between the STM tip and the semiconductor surface.