PURPOSE: To develop a robot-arm type image-guided surgery system for percuatneous screw fixation of the sacro-iliac joint and to evaluate its accuracy. MATERIALS AND METHODS: We have developed an image-guided surgery system using a three-dimensional digitizer (Microscribe 3-D G2, Immersion, USA) and a personal computer. The registration error and target localization error at fiducial registration were measured 30 times for each using a phantom made with plastic pelvic bone model (Sawbones, USA). Sixteen 6.5 mm cannulated screws were inserted into four plastic bone models, and the accuracy was evaluated. RESULTS: The target localization error was 1.46±0.47 mm while the registration error was 0.73±0.23 mm. All of the 16 screws were inserted well across the sacro-iliac joint, and there was neither cortical breach nor collision between screws or washers. CONCLUSION: The accuracy of the developed system was similar to that of optical tracker-based navigation systems, and its helpfulness and usefulness was proven with simulation surgery using plastic bone models.