An underfilm corrosion simulation method for epoxy coated steel panels in ship’s water ballast tanks (WBT) has been developed. The incubation and extension of coating failure are simulated by using 2-dimensional cellular automaton method, and the steel diminution is simulated by modifying the corrosion model proposed by Kawamura et al. (2015). Analysis parameters are determined by using the results of onboard exposure and cyclic corrosion tests performed by Shiotani et al. (2014). The change in corroded surface shape of epoxy coated and scribed steel panels made of conventional steel and corrosion resistant steel (CRS) exposed in an ore carrier’s WBT for 4.8 years is simulated. The simulated coating deterioration (blister) area and the corroded surface profile agree well with those measured. This demonstrates the effectiveness of the developed simulation method and the determined parameters. The differences in analysis parameters between conventional steel and CRS suggest that CRS can reduce the harmful effect of the active corrosion region on the remaining coating life at the blister’s frontline and the corrosion under the blister.