The surface penetrant is performed for concrete structures as one of the prevention maintenance. The chloride ingress at the reform part in the surface area is different from that at the deeper part where the concrete is not reform. However, there is no established method for determining the chloride ion diffusion coefficient at the reform part.<br> From the above backgrounds, this paper proposes a method for determining the apparent chloride ion diffusion coefficient in an outer layer reformed by silicate type surface penetrants. First, the part reformed with the surface penetrant was investigated from the Vickers hardness distribution. Subsequently, the concrete specimen applied the surface penetrant was exposed to the chloride environment, and the chloride ion concentration distribution was measured. Simultaneously, the chloride ion concentration distribution was measured for the concrete specimen without any penetrant. Thereafter, the chloride ion concentration distributions for the both cases were compared. To determine the diffusion coefficient at the reform part, the cover concrete with the surface penetrant was increased by considering the equivalent cover depth. Herein, the equivalent cover depth implies that the tegmental thickness with the low diffusion coefficient by the surface penetrant is replaced with the cover depth of normal concrete. As a result, the apparent chloride ion diffusion coefficients at the reform part by the four types of the silicate surface penetrant could be estimated.<br> Furthermore, an actual application scenario was demonstrated the effect of delaying the corrosion occurrence time after the surface penetrant was performed as the prevent maintenance. Therefore the chloride ingress was calculated by the investigated reform part and the estimated apparent chloride ion diffusion coefficient in the trial. The results confirmed that the use of the surface penetrant delayed the occurrence of corrosion by 12 years.