Abstract: An apparatus for detecting defects in a moving cast metal strip is provided that includes at least one Lamb-wave transducer housed inside a sensor wheel made of flexible material. The sensor wheel contains an acoustic coupling fluid and is designed to come into running contact with the surface of the product to be inspected. A runner is placed upstream of the sensor wheel for contact with the surface of the strip to be inspected. Upstream of the runner, a delivery system for delivering coupling fluid onto the surface of the moving product is provided, this fluid serving to wet the surface of the strip. The runner includes a roller having a contact width the same or greater than the width of the product for uniformly spreading the coupling fluid over the product. All of these components are installed in a movable carriage which is designed to run over the metal strip surface. The apparatus is capable of detecting defects in a steel strip moving at high speed.