A device for monitoring the position of an oil/water contact (OWC, 22) between an oil-continuous fluid (2o) overlying a water-continuous fluid (2w) inside a casing pipe (7). The device includes a transmitter (5) for a generating an electromagnetic signal (ST) and the transmitter (5) is provided with electrical energy (GT) from a voltage signal generator (G). The transmitter (5) is arranged inside the casing pipe (7) in the oil-continuous fluid (2o) and above the oil-water contact (22). The electromagnetic wave signal (ST) partly propagates downwards from the transmitter (5), is partly reflected from the oil-water contact (22), and is partly reflected by the end of the casing, giving rise to an upward propagating, reflected electro-magnetic signal (SR). The device also includes a sensor (6) for detecting the reflected electromagnetic signal (SR), and the sensor (6) is also arranged above the oil-water contact (22), providing a sensor signal (RR) to a receiver (60) for receiving the sensor signal (RR) and further to an analyzing device (61) for analyzing the sensor signal (RR), e.g. with respect to two-way propagation time or amplitude in order to calculate an elevation level for the oil-water contact.