Modified tone decay test(Rosenberg's procedure) was carried out for 20 cases of normal ear, 20 cases of conductive hearing impairment and 40 cases of sensori-neural hearing impairment. The results were as followings ; 1) No tone decay was observed at all frequencies in the cases of normal and conductive hearing impairment and in the cases of sensori-neural impairment, the amount of tone decay was 1.6 dB at 1,000 Hz., 4.1 dB at 2,000 Hz. and 7.6 dB at 4,000 Hz.. 2) In the case of cerebral tumor, significant amount of tone decay was observed ; 20 dB at 2,000Hz. and 25 dB at 4,000Hz.. 3) The case which the amount of tone decay exceeded 15 dB, was not observed in normal and was only one(5%) at 4,000Hz. in the cases of conductive hearing impairment. And the cases in which the amount of tone decay exceeded 15 dB among the sensori-neural impairments were one case(2.5%) at 500Hz., 2 cases(5%) at 1,000Hz., 4 cases(10%) at 2,000Hz. and 13 cases(42.5%) at 4,000Hz.. 4) The cases in which the amount of tone decay exceeded 15 dB among the sensori-neural impairments were as followings ; a) Two cases of acoustic trauma ; One at 2,000 Hz. and the other at 4,000Hz.. b) 4 cases(57.1%) at 4,000Hz. in the cases of presbycusis. c) One case(12.5%) at 4,000Hz. in the cases of Meniere's syndrome. d) 11 cases of undetermined origin ; One case(5.5%) at 500 Hz., 2 cases(11.1%) at 1,000Hz., 2 cases(11.1%) at 2,000Hz. and 6 cases(33.3%) at 4,000Hz.. Consequently, 13 cases of sensori-neural impairment showed over 15dB decay at 4,000Hz. ; among them 10 cases were presbycusis and undetermined origin. One case of acoustic trauma and 2 cases of undetermined origin showed over 15dB decay at 2,000Hz. Over 15dB decay at 500Hz. and 1,000Hz. was observed only in the cases of undetermined origin.