[en] The dynamical behavior of an elastic control rod inserted into a vibrating rigid thimble, which is filled with a viscous fluid, was studied in a theoretical manner. In this problem the excited mode shape of the rod varies unsteadily with the time elapsed, because the span of the rod increases during vibration. The maximum deflection of the rod becomes small, if the higher mode appears with the insertion of rod. Although only a linear model was treated in this study, the position of the first collision between the rod and the thimble was examined for a relatively higher input acceleration. The time required for the insertion of the rod increases as the input acceleration increases. But its increasing rate is very small, because the effect of fluid force on the falling motion of the rod is small compared with those of the dead weight of the rod. (orig.)