A room-temperature wet process which allows control of the wavelength of photoluminescence from porous silicon has been developed. This process reduces the size of the silicon crystal by the repeated formation and removal of the surface oxide. It has been learned that the surface after this process does not have residual oxide. Synchrotron radiation photoemission spectroscopy and X-ray absorption near edge structure measurements have revealed that the bandgap of porous silicon becomes wide with repetition of our wet process.