An analytical method to calculate motions of an elastic floating mat-like structure in wave is developed, using the method of velocity potential continuation to analyze the fluid region and thin plate theory. The deflections of the plate is approximated by an expansion in terms of the natural mode functions for free vibration of a free-free beam. Calculation method for steady wave drifting forces are also shown by using Maruo's far-field theory. To verify the accuracy of presend method, calculated results for a rectangular plate are compared with the experimental results. The Calculated and measured amplitudes of the deflections and bending moments of the plate are in good agreement.