Abstract: An apparatus for testing hardness under load has a measuring sensor in the form of a manual apparatus in which a mobile rod-shaped resonator can be slidingly inserted. The free end of the resonator, situated outside the housing, has a measuring tip. The resonator is held against the housing of the measuring sensor by a spring and is linked to ultrasonic converters connected to an electronic generator and to a receiving circuit which records changes in the duration or frequency of the vibrations of the generator. A basic apparatus comprises a display unit and is connected by a connecting line to the measuring sensor. A memory for storing individual parameters of the rod-shaped resonator and the electronic generator is arranged in the housing of the measuring sensor. The basic apparatus is detachably connected by insertion to the measuring sensor.