Liquefied natural gas (LNG) has been widely used as a substitute fuel for commercial purposes. It is transported mainly by LNG ships which have primary and secondary leakage barriers. The former is composed of welded thin stainless steel or invar plates, while the latter is composed of adhesively bonded glass composite or aluminum foil sheets. The role of the secondary barrier is to maintain fluid tightness when the primary barrier fails during the transport of LNG. The tightness of the secondary barrier is dependent on the wetting characteristics between the adhesive and adherend of the bonded structure during bonding operation, which depends much on the contamination on the adherend surface. Therefore, in this work, an optical measuring device of oil contamination on the aluminum surface for the secondary barrier was developed. A transparent oil was used as the contaminant and its effect on the bonding strength was investigated. From the experiments, it has been found that the developed measuring device for oil contamination can be used to detect oil contamination on a large bonding area of the secondary barrier in ship building yards.