In this paper, the problem in the CO2 infrared laser medical operation device that propagates the laser in the hollow optical fiber was dealt. There is risk of the accident of damaging by a fire when leak is opened in the hollow optical fiber, and the laser light is discharged outside. This study is the detecting method for the leak on hollow optical fiber by using the sound. A basic experiment concerning this method and comparison result between the theoretical analysis and the experiment are reported. The leakage of the narrow tube with large attenuation can be detected. The experiment was corresponding to the tendency to the theoretical analysis for the presence of the leakage and the change in the hole diameter. This method is inexpensive than the method of adding the optical device, it makes small hand peace, and doesn't ruin handling that is the advantage of the hollow optical fiber. © 2011 The Japan Society of Mechanical Engineers.