Very large floating structures (VLFS) are being considered for various applications such as floating airports, offshore cities and so on. A VLFS (Mega Float I) with length of 5 km, breadth of 1 km, and draft of 1 m was proposed by The Technological Research Association of Mega-Float in 1996. The elastic deformation of these structures in waves is important, because vertical dimension is small compared with horizontal. There are some numerical methods proposed to estimate its elastic behaviours and it is needed to validate these methods comparing with model test. Therefore, the authors carried out the tank test focusing on elastic response in wave using 1/100 scale model (VL 50) with 50 m length. The model was designed on the basis of the law of similitude and has elastically similar bending rigidity and weight with the full scale structure. The vertical displacement distributions are measured at 126 points along the center line by potentiometers, and the strains at 10 points on the upper surface at weather side and lee side respectively. Some results of vertical displacement showed clearly long periodic deformations with the several node superposed on short periodic deformations that can be considered the cause of interaction between the model and the side walls of tank. The elastic behaviors of VL 50 in wave are computed by the analysis method based on FEM and the pressure distribution method using the zero-draft Green function. The effect of the side walls are dealt as symmetric refraction. The calculated results agree qualitatively with the experimental results. The side wall effect can be explained well by the present method.