A pontoon type structure was recently highlighted since The Floating Structures Association of Japan proposed the new concept as the most beneficial one in 1994. The structure is supposed to be mat-like structure with length of 4 to 5 kilometers and depth (draft) of 5 (1) meters. The concept was further developed by The Technological Research Association of Mega-Float which was established in 1995. They constructed 300 meter long prototype structure to carry out at-sea test. The authors carried out tank tests to investigate hydroelastic response of the prototype structure using its scale model. The research was undertaken partly as joint work with T. R. A. of Mega-Float. The scale ratio of the model was decided as 1/30, with length of 9.75 meters, breadth of 1.95 meters and draft of 1.66 centimeters. The model was designed on the basis of the law of similitude and has elastically similar bending rigidity with 300 meter long prototype structure. A series of tank tests had been conducted focusing on the elastic response of the model in regular waves. The vertical motions have been detected through potentiometers and the bending moment through strain gages both distributed on the upper surface of the model. The analysis method based on 2-D structure and 2-D fluid modeling has been established in which the structure is approximated as a rectangular plate and the hydrodynamic characteristics is evaluated using the pressure distribution method based on the zero-draft Green function. The equation of motion, in which the interaction between structure and fluid is taken into account, is solved by the direct procedure instead of by modal analysis. The calculated results show quite satisfactory agreement with the experiment and the study has highlighted some remarkable characteristics concerning hydroelastic behavior.