A plunger type of wave-absorbing-wave-maker was newly constructed in the wave tank of the National Defense Academy. The wave maker is controlled for absorbing reflected wave by the additional signal throught three digital filters, two of them for separating reflected wave by using two wave sensors and one for predicting the propagation time of the reflected wave and making wave cancelling the re-reflected wave from wave-maker. This idea of time-domain method for separating incident and reflected waves is due to Frigaard et al.(1994). The performances of absorbing reflected wave in periods 0.6 to 1.2 seconds are checked in some experiments by terms of re-reflected wave heights from the wave-maker, changes of added mass and dumping coefficients and the other values for a semi-submerged elliptic cylinder in both cases when fixed in waves and oscillated in sway motion in calm water and in the other cases. The absorbing efficiency is about more than 80% when the wave-maker is acting only for abosrbing incoming wave radiated from a oscillating floating body and more than 70% when for making incident wave. This discrepancy of the efficiencies is concluded to be mainly due to nolinear reflection properties of moving plunger.