Abstract: Microwave device for the continuous measurement of the viscosity of a viscous medium. The device includes a microwave source connected via a microwave guide to a sensor essentially incorporating at least one semi-rigid, coaxial microwave guide (2) for immersion in the medium. The cable has an outer sheath (4) and a core (6), which conduct electricity and are arranged coaxially. The space defined between the outer sheath and the core is filled by a dielectric (8). The core has a bare end (12) for placing in contact with the viscous medium. The sensor is intended to produce in the medium incident microwaves of a given power level, and to detect the power of the microwaves reflected by the medium. A second microwave guide (10) is provided for coupling the detector to the sensor.