ADC12 is one of the common aluminum alloys for automobiles because it has suitable for casting and machining. However, the corrosion resistance of ADC12 is insufficient in comparison with other aluminum alloys. The corrosion depends on chemical composition of aluminum and circumstance around aluminum. It was considered that a crevice such as a seal gap accelerates corrosion rate. Therefore, the corrosion at a sealing gap between ADC12 and rubber gasket was investigated. Salt water corrosion tests were carried out with an o-ring compressed between ADC12 plate and plastic plate. Corrosion depth and corrosion area at sealing surface were measured with a microscope. The corrosion depth at the sealing surface was deeper than that outside it. Since smooth surface of aluminum prevented the sealing surface from corrosion, it was considered that the narrow sealing gap enabled to decrease in the corrosion rate.