The sensor, to determine the concentration of glucose in blood, is implanted within an ampoule (1) containing a sensitive liquid. Glucose can penetrate into the ampoule through its cellulose mantle (2), where the viscosity of the liquid and glucose mixture is measured. A portable instrument, placed externally on the skin, controls the measurement and evaluation of the readings. The sensor, to determine the concentration of glucose in blood, is implanted within an ampoule (1) containing a sensitive liquid. Glucose can penetrate into the ampoule through its cellulose mantle (2), where the viscosity of the liquid and glucose mixture is measured. A portable instrument, placed externally on the skin, controls the measurement and evaluation of the readings. The measurements are taken from the oscillations of an oscillating body (8) supported by a plastics cylindrical section (3) within the ampoule, excited by a magnet (6) in the ampoule. The oscillating behavior is analyzed from the fading behavior of the body oscillations after the magnet is switched off.