A novel device for the absolute determination of the current of an ion beam without any deterioration of this beam was constructed. It uses the fact that the molecules of the residual gas in the beam line are ionized by the beam. These slow ions can be counted by means of a channel-plate detector. Because the ionization probability is proportional to the number of beam particles impinging a certain gas volume this counting rate is a direct measure of the beam current. By comparison with the integrated charge, measured with a Faraday cup behind this device, an accuracy of better than 3% for the determination of the absolute intensity of the ion beam can be achieved.