The invention relates to a diagnostic apparatus with which diseased hard tissue, in particular carious dental tissue, can be examined through a small working channel, to see whether it can be made healthy again by remineralization. This diagnosis apparatus tightly (54) seals the working channel, produced in the tissue, from its environment, and places the cavity in the tissue under pressure, after which the pressurization is ended and a pressure drop is measured by means of a pressure measuring instrument (60). The speed of the drop in pressure is a measure of the extent of the changes to the diseased tissue. The invention also relates to a treatment device for supporting the remineralization or reinfiltration of diseased hard tissue and for preparing this, as well as to diagnosis and treatment agents and treatment substances.