In measuring the heel angle of a ship under her inclining experiment, she is generally inevitable to oscillate more or less owing to the external forces such as wind and wave. Even in such a case the measuring method by a long pendulum which has its own oscillation is generally still in use. Therefore, when accuracy is especially required, it may be said that, if the above method being taken, it will be very difficult to distinguish exactly the small heel angle induced by the moving weight. Now the authors have lighted upon the optical method by a camera in place of a pendulum to measure the heel angle of a ship in such a oscillating case. This optical method will be very simple and accurate. In this paper, general investigations in camera are described together with some test results with four actual ships obtained by our optical method. From these test results you may also perceive that the exact rolling period of a ship can easily be obtained, if necessary, without the rolling test.