The research of VLFS (Very Large Floating Structure) for various applications such as floating airports is making a steady progress by the Technological Research Association of Mega-Float and others. The pontoon type structure with the length of 5 km, breadth of 1 km and draft of 1 m can be expected in the concept. This structure will be surrounded by the breakwaters for decreasing the influence of waves. Therefore the floating units of several hundred meters long should be connected together on offshore site instead of the construction in the dockyard. This paper deals with the wave induced connecting loads and the elastic behaviors of the box-shaped floating unit divided into three parts longitudinally. The 300 m long prototype structure used for atsea test by the Technological Research Association of Mega-Float has been modeled for the experiment. Three kinds of connecting conditions such as free condition without connection, pin-connection and beam connection have been considered. The numerical analysis code has been developed to estimate the elastic motions and the connecting loads. The analysis is based on FEM and the pressure distribution method using the zero-draft Green function. The authors have also carried out the tank tests using a 1/30 scale model for the validation of it. The test model has been designed on the basis of law of similitude concerning the stiffness and the mass of the structure. The calculated results show a quite satisfactory agreement with the experiment. It has been confirmed that the method has a sufficient accuracy for the practical use.