The wave dissipating caisson with a permeable front wall and a wave chamber has the advantage of low wave reflection from the vertical fave. However, various factors related with wave and structural conditions influence on the refluection coefficient.
　A theory is presented to calculate the reflection coefficient for the wave dissipating caisson with a vertical slits in its front wall. The head loss coefficient and the effective inertia length used in the theory are formulated on the basis of systematic experimental results. The coefficient of correlation between the refluection coefficients by the theory and experiments is 0.907, and 70 percents of the experimental data fall within the range of ±0.1 in the absolute value from the calculated reflection coefficient.
　The optimum dimensions of the width and the depth of wave chamber and the opening ratio of the slit wall to make the refulection coefficient minimum for a given
wave condition are discusses according to the theory.