The behavior and velocity of falling rubbles in quiescent water depends on their weight, specific gravity, diameter, shape etc.
　We conducted some falling experiments on falling velocity and drag coefficient of the rubbles, classifying the rubbles by their weight (5g～1t) and flatness ratio (0.1～0.8) as a shape function into the groupe.
　In this experiment we confirmed that falling velocity is in proportion to the sixth root of their weight, and decreases rapidly with increaseing their flatness ratio.
　We also found that drag coefficient increases rapidly with increasing flatness ratio.
　We measured falling velocity of the rubbles as a groupe (5 and 10), but their velocity is not different from that of individual rubbles.