This paper deals with the dynamical stability of a ship equipped with an anti-rolling tank in accordance with the Japanese Standard of stability for ships. That is to say, both in case of C₁ criterion coefficient, (ship, which is going under the condition of steady wind and steady waves, is subjected dynamically to a gust of wind having 1.5times wind pressure of the steady wind when the ship is at the maximum angle of roll windward) and in case of C₂ criterion coefficient (a ship, which going under the condition of steady wind only, is subjected dynamically to the similar gust of wind as in case of C₁ coefficient), the maximum angle of roll is calculated theoretically. Now the present authors suggest how to treat the anti-rolling tank so far as the rule of stability for ships is concerned. Besides, they have also gotten the maximum angle of roll both theoretically and experimentally in the case when a ship is subjected to an impact force, thereby checking the theoretical calculation.