Abstract: For the purpose of forming a reinforcing layer of a decay-retarding agent on the surface of teeth, a device is provided for transmitting ultrasonic waves to the teeth while the latter are in contact with a decay-retarding agent. The device includes an enclosure which is adapted to contain a medium for transmitting ultrasonic waves, this enclosure including a sheet material which defines a hollow interior space in which an ultrasonic wave transmitting medium is adapted to be located. This sheet material of the enclosure has the property of being impermeable to the ultrasonic wave transmitting medium, so that the latter cannot pass through the sheet material. In addition, the sheet material has the property of transmitting ultrasonic waves therethrough.