Rodents are often used in medicine, brain science, pharmacology, and psychology, as experimental animals. We have been proposing a novel method that involves usages of mobile robots as agents to interact with animals to understand animal behavior. A robot which is able to locomote not only on floors but also walls are strongly expected to increase the variety of behavioral experiments. The purpose of this study is to develop a new mobile robot which is able to locomote both on floor and on walls interacting with rats. For this purpose, we invented a novel mechanism which made a mobile robot locomote by wheels on iron walls. The mechanism consists of an active bending joint in the body, a passive tilting magnet, and a passive tilting tail. We implemented this mechanism into a new small mobile robot, named WR-7 (Waseda Rat No. 7). In this paper, proposed mechanism and its implementation are described.