To assess safety, cracks in concrete structure are measured by inspectors, who observe cracks with their naked eyes and record them. But manual inspection is slow, and measured crack data is subjective. Therefore, this study proposes inspection system for measuring cracks in concrete structure and providing objective crack data to be used in safety assessment. The system consists of the mobile robot system and crack detecting system. The mobile robot system is controlled to keep a constant distance from the wall to acquire image data with a CCD camera on scanning along the wall. The crack detecting system extracts crack information from the acquired image using image processing. To improve accuracy of crack recognition, the geometric properties and patterns of cracks in a structure were applied to image processing. The proposed system was verified with experiments in both laboratory environment and field.