Measurements on the dynamical forces by the water in a tank and the stresses induced by them were carried out on an ore carrier. (The ore carrier was used in order to avoid danger of gas explosion caused by measuring instruments.) Measuring items are the accelerations of ship motions, the dynamical pressures by ballast water and the stresses due to them. Results are as follows. (a) The motion of the water is very dull in the tank which has many transverse members in itself such as side tanks. (b) But in the tank located at fore part of a ship, the motion of water is considerably larger than in the midship tank. (c) The load on a swash plate is much smaller compared with its strength. (d) At slots of a horizontal girder web plate, there may occur tearing forces of remarkable magnitude in a severe case.