An apparatus for detecting biofilm in the water conduits of dental units, especially biofilm adhering to the inside surfaces of the conduits, can be used on a water line (1) equipped with a plurality of conduits (2) for supplying fluids to handpieces (3) and fluid consuming units (4) that use fluid from a main supply (5) or accessory fluids from corresponding independent lines (6). A portion (7) of one of the conduits (2) is equipped with means (8) for detecting the presence of the biofilm on the surfaces (7a) of the portion (7) itself. The invention also relates to a method for detecting the biofilm. The method comprises the steps of contacting the biofilm attached to surfaces (7a, 9) with a reagent substance or fluid; altering the biofilm by the reagent substance or fluid; and detecting the alteration that has taken place in the biofilm using the detecting means (8).