The hearing tests were made on 4372 persons working under noise in a coal mine to investigate and analyze the relationships among the rates of occurrence of C₅ dip and the changes of age, the number of working years, and the noise intensity in the working places. The results were as follows : 1) C₅ dip was found in 411 persons (9.4%) of 4372 workers. 2) The older the age, the higher the rate of occurrence showed the increasing tendency. 3) The longer the number of working years, the higher the rate of occurrence showed the increasing tendency. 4) The grade of C₅ dip had a tendency to vary from slight to worse in proportion to number of working years and the changes of age. 5) The distribution of high grade C₅ dip and low grade was nearly pararell each other in the age group, but was not in the group years service. 6) The occurrence rate and the changes of grade were highly related to the age than the number of working years. 7) The rate of occurrence of C₅ dip was remarkably related to the noise intensity of the working places in the group kind of service.