Degenerative changes in the morphology of the stapedius muscle caused by aging using a collection of human temporal bones obtained by our department. The diameter of the muscle fibers, the variance in muscle fiber diameter, the ratios of intermuscular connective tissue and muscle to the area of the bone niche for the stapedius muscle, the number of muscle fibers around the tendon, and the adipose cell count were measured for each temporal bone. In total, 80 temporal bones were examined, excluding specimens with facial nerve degeneration, degenerative diseases, middle ear disease and leukemia. Between the ages of 38 fetal weeks and 9 years, the muscle fibers and thin and the variance in fiber diameter is small. The areas of intermuscular connective tissue, muscle and the number of muscle fibers around the tendon do not change between 38 fetal weeks and 60 years. All of the parameters measured showed no significant changes between the ages of 10 and 60 years. The variance in muscle fiber diameter began to increase in specimens aged 70 years or older because some of the muscle fibers had begun to atrophy. Adipose cells were sometimes observed in very old specimens. In contrast to the tensor tympani muscle, in which degenerative changes begin during childhood, atrophic degeneration of the stapedius muscle as a result of aging begins at a very late age. The difference in aging between the stapedius muscle and the tensor tympani muscle is probably related to evidence suggesting that the stapedius muscle contracts in response to acoustic stimulation.