More than 30 years have passed since the steel plate bonding technique was applied for the first time to reinforced concrete (RC) slabs of road bridges in Japan as a strengthening measure against fatigue damage. Debonding of the steel plates as well as damage in concrete are often found as a dull sound during hammer tests. Efforts are being made for more frequent inspections; however, visual observation has a limitation in detecting damage in the concrete of the repaired slabs which are covered with steel plates at the bottom. Although the presence and extent of debonding can be roughly known from the change in sound during hammer tests, there are no decisive methods to detect or evaluate the internal damage of the concrete. The purpose of this study is to establish a method to evaluate damage in concrete of steel plate-bonded RC slabs on road bridges in service, with the focus placed on the temporary set anchors in the repaired slabs which had been used to hold the steel plates until settlement of adhesive agent for secure bonding. The proposed technique utilizes the anchor bolts as ultrasonic probes.