A device for exposing teeth to radiation of ultrasonic waves while the teeth are contacted by a decay-retarding agent, for forming a reinforcing layer of the latter agent on the teeth. In addition to a generator for generating ultrasonic waves, the device includes a support structure which carries a plate capable of transmitting and reflecting the ultrasonic waves. The latter plate is situated between the generator and teeth toward which waves are generally directed from the generator, so that the latter waves are received by this plate which transmits part of the waves through the plate to the teeth at the side of the plate opposite from the generator while another part of the waves are reflected by this plate away from the teeth which receive the transmitted waves, so that in this way the reflected waves may be utilized to treat additional teeth.