Dynamic collapsing of side structures of an ore carrier under non-impulsive wave pressure is investigated theoretically and experimentally. The damages due to impulsive wave pressure are restricted to local ones such as dents of shell plates, because the duration of peak pressure is very short and consequently the momentum is small in spite of its high peak value. On the other hand, non-impulsive wave pressure may cause serious damages in side structures widely because of its long duration and its large momentum, though it has comparatively small maximum value. In the present paper, the following items are clarified : (1) Non-impulsive wave pressure may yield vital damages in an ore carrier, if it goes beyond the maximum strength of the wing tank structures. (2) The amount of deflection of the side shell caused by non-impulsive wave is almost two times as much as that calculated by the statical analysis. Dynamic response of side structures in oil tankers may be studied along the same line of thought.