In Part I of this paper, the author summarized the general character of the response of ship oscillations among waves, especially of the rolling. In Part II, he discussed the problems which appears on the way to analyse the irregular oscillations of ships on the irregular ocean waves, as stochastic processes. In this Part III, the author discussed the meaning and the purposes of the model experimental tank test which is performed in an irregular wave system produced in the tank by an ingenious method, inaugurated by Prof. E. V. Lewis. The author introduced the concrete method of test and several notices, which he thought to be some help for others to do this kind of works. In doing this, the author used the results he has obtained at the Davidson Laboratory, Stevens Inst. of Tech., from the series of rolling test ih irregular tank waves as an example. He also used this results to check the validity of the analysing method, he has mentioned in Part II, as well as to show the fact that the same character of ship rolling response in regular waves, that he mentioned in Part I, are kept to be true in irregular waves too.