A locomotion ability of robots has been studied to move on a various terrain stably. We are developing a robotic wheel which locally transforms its diameter to improve the mobility on an uneven terrain. The robotic wheel locomotes on a flat road using the high mobility of a wheel, and also on rough terrains by changing its diameters. We introduce a variable diameter wheel as a prototype of the robotic wheel, and investigate its availability. Without installing an axle motor, the robot is able to locomote by utilizing the extension and returning of the diameters by moving its centre of gravity. The extension of the spokes works to push the robotic body in order to locomote, and the returning of the spokes make the wheel get the rotation momentum. The extension and returning mechanism is driven by rack pinion mechanism. We verified its mobility through four experiments, which are the testing locomotion on flat ground, the climbing over the step, controlling the robotic wheel by IMU, and the braking by the extension of spokes. The trajectory which the tip of the spoke draws is also compared with the simulation using MATLAB.