Thermal donors in undoped germanium were produced by heat treatment at 350℃ for 200h, and the level position for the thermal donors was tentatively estimated to be 0.36 eV from the conduction band. When the sample was subsequently implanted with 25 keV hydrogen,concentration of the level associated with the thermal donors was drastically reduced and the position of DLTS peak corresponding to the level shifted to lower temperature. The restoration of the neutralized thermal donor electrical activity began after a 15 min annealing at 200℃ and essentially completed after annealing at 280℃.