A capacitive action sensor unit for monitoring the quality of fibre material (1) is disclosed. The sensor unit comprises a measuring electrode (2) curved to form an arch in the transport direction (X) of the fibre material (1), the upper side of which is trough-shaped, a transmitting electrode (3) at a separation from and facing the upper side and a screen electrode (4) surrounding the underside of the measuring electrode (2). An AC generator (5) is connected between the transmitter electrode (3) and the screen electrode (4), and the measuring electrode (2) is connected to an analytical circuit (8).