We report on the optical determination of the carrier-density profiles near surfaces in SrTiO3 crystals by means of photoluminescence (PL) dynamics measurements. The PL dynamics under band-to-band excitation depends strongly on the excitation photon energy for different optical penetration depths. In nondoped and Ar+-irradiated SrTiO3 crystals, we evaluate the depth profile of carriers near the surface based on the lifetime of the Auger recombination of electrons originating from oxygen vacancies with photocarriers. Our PL spectroscopy clarifies that in nondoped SrTiO3, the near-surface oxygen-deficient region is a few tens of nanometers in depth.