Abstract: The apparatus of the invention has two heads each of which has three sensors for measuring the propagation speed of ultrasonic waves, the degree of moisture, and the temperature in a test material, in conjunction with a module having a microprocessor and memories. The microprocessor calculates the measured propagation speed of the ultrasonic waves. This speed is normed with respect to a set temperature (e.g., 20.degree. C.) and a set value (e.g., 15%) of the degree of moisture. From these measurements, the microprocessor searches for the class of standardized mechanical properties in which the material falls. The microprocessor finds the class in which the value of the normed propagation speed falls in a table of values contained in one of the memories. The apparatus thereby enables the determination of the class of the material automatically and non-destructively.