For ship safety, periodical inspection and maintenance in service is very important together with care during designing, building, and operation. For this reason Mitsubishi Heavy Industries, Ltd.(MHI) has developed an inspection robot system that can carry out easily, safely, and accurately tank inside construction inspections of the crude oil tanker, which has been difficult to carry out from the restriction of height, large spaces, and residual sludge. Its features are as follows : (1) Automatic setting of monorails makes scaffolding as high as 30 m unnecessary and brings labor saving and efficiency improvement. (2) By means of remote/automatic operation from the upper deck, pretredtment tools, television camera for remote visual inspection, and sensors for cracks and plate thicknesses can access to narrow construction areas and very safe and highly-accurate inspection can be achieved.