Abstract: Device for the continuous measurement of the impregnation level by a substance of fibers which may or may not conduct electricity. This device has a cylindrical resonant cavity (2) excited in accordance with mode TE.sub.113 and provided with first microwave input and output (e1, s1) for respective coupling to a microwave source (2) and a detector (16) of the microwave power from the cavity when the fiber is conductive, so that the first component (E1) of the electrical field of the resonant mode is perpendicular to the fiber, second microwave input and output (e2, s2) for respective coupling to the source and the detector when the fiber is non-conductive, so that the second component (E2) of the electric field of the resonant mode is parallel to the fiber, a mode filter (40) for eliminating from the cavity the first component of the electric field when the fiber is non-conductive, and means for processing the signal supplied by the detector in order to deduce the impregnation rate therefrom.