Perforation corrosion of steel sheet lapped joints such as door hems is one severe corrosion type taking place on automobiles. This is because the lapped joints are exposed to a corrosive solution containing de-icing salt during driving and the solutions flowing into the crevice at the lapped joints, causing severe corrosion. Therefore, it is quite important to evaluate corrosion degradation of steel at the crevice of the lapped joints. In this study, effect of crevice geometry on corrosion behavior of carbon steel is investigated by immersion tests in 5wt% NaCl solution. In order to mimic steel sheet lapped joints, a block jig made of ABS resin having a cylindrical crevice former was prepared by a 3D printer. Crevice geometry on carbon steel such as a gap between a steel and a jig, and a diameter of the crevice former was changed by employing the jigs with different geometry. It was found from the results of the immersion tests that steel corrosion mainly took place below the crevice former and little corrosion was observed on the outer part of the steel apart from the crevice former. Furthermore, weight loss of the steel sheet during the immersion tests decreased with increasing the diameter of the crevice former. These results suggested that steel corrosion proceeds under the crevice former by the differential aeration inside and outside of the crevice, forming anode and cathode areas on the steel surface in the solution.