Colonoscopy is the most effective procedure for cancer screening. However, insertion of a colonoscope into the large intestine remains challenging because of the complex and highly variable anatomy of the colon, particularly variations in its inner diameter. Various self-propelled colonoscopy robots have been proposed to overcome these difficulties. Nevertheless, several existing systems require complex mechanisms for adaptation to changes in the inner diameter of the colon, which increases structural complexity and limits practicality. Therefore, simpler and more adaptive propulsion mechanisms are required. In this paper, we propose a self-propelled colonoscopy robot with an elastic wall-fitting mechanism. This mechanism utilizes the elastic force of shape memory alloy (SMA) wires to adaptively conform to variations in the inner diameter of the colon, ensuring continuous contact with the intestinal wall. A prototype robot was fabricated using a 3D printer. Evaluation experiments were conducted using simulated intestinal tract models with inner diameters ranging from 30 to 60 mm to assess propulsion performance and thrust generation. The experimental results demonstrated that the proposed robot successfully self-propelled in all the simulated intestinal models. The robot achieved a maximum propulsive speed of 11 mm/s and generated sufficient thrust across the diameter range tested, indicating stable propulsion performance despite changes in the inner diameter. These results confirm that the proposed elastic wall-fitting mechanism effectively adapts to variations in the inner diameter of the colon without requiring complex mechanical structures. By leveraging the elastic properties of the SMA wires, the robot maintained consistent wall contact and stable propulsion, suggesting its potential applicability in safe and efficient colonoscopy procedures.