A robot capable of moving on a vertical or overhanging wall has been expected for inspection use of the outer walls of tall buildings, dams, and some other vertical walls. For the purpose, the sucking force produces by the negative pressure in the suckers is available to sustain the robot on the surfaces. To adopt a robot to the various types of irregular surfaces, a walking mechanism is suitable. A bipedal walking model which has a sucker on each leg is considered. When it is walking on a vertical wall, a sucker has to sustain the whole weight, then it is very important to examine its safety condition. To develop a control system of walking, a numerical simulation is carried out and safety of walking is investigated. By these studies, a model was constructed and tested. The safety of walking on a vertical wall and transition from the wall to the ceiling was confirmed by the experiments.