Secondary ion mass spectrometry (SIMS) has been used to study the formation of surface double layer on the cleaved (100) surface of a NaCl single crystal at ambient temperature. According to the theory of Kliewer and Koehler on the surface double layer of the alkali halide crystal surface, an excess amount of doubly ionized impurity ions should exist beneath the crystal surface in contrast with the deficiency of positive ion vacancy there at room temperature. The theory also predicts that the concentrations of these two species of point defects are equivalent in the interior of the crystal, thus assuring the electrical neutrality. Cleaving of well-annealed crystal creates diffusional fluxes of point defects toward the surface. Experimental results show that the acculumation of impurity ions onto the crystal surface is indeed in progress after cleaving, thusk substantiating the Kliewer and Koehler theory. Numerical calculations to solve the diffusional equations on impurity ions suggest that new potential of the type A exp[-Bx²] must be introduced to its chemical potential. Here x is the depth from the surface, A and B being constants.