The steel plate bonding method has been conventionally used on reinforced concrete (RC) slabs of road bridges in Japan as a protection from fatigue damage. As more than 30 years have passed since the first application of this method, some of the RC slabs previously repaired by steel plate bonding started to show debonding of the steel plates (detected by dully sounds) or internal damage of concrete of the slabs. A steel plate-bonded RC slab forms a composite structure of steel and concrete. Its flexural rigidity and other capacities will decrease if the adhesive agent or other material combining the steel and concrete is deteriorated. If the concrete slab itself is damaged significantly, the resistance performance of the composite structure will be affected, irrespective of the bonding condition. However, although the presence or extent of debonding of adhesive agent can be roughly estimated from the change in sound observed during inspection by the hammer impact test, there are no decisive methods for detecting or evaluating the internal damage of the concrete. This study proposes a method for evaluating damage in concrete of RC slabs bonded with steel plates on road bridges in service, using the anchor bolts, which had been installed for holding the steel plates temporarily until complete setting of adhesive agent, as ultrasonic probes.