A steady numerical computation of random wave is important to the safe and economical design of a port structure. The energy balance equation is widely used as fundamental equation for the numerical computation of the random wave transformation. This method is applied in the relatively small harbor with uniform water depth. Recently, the research considering the effect of wave diffraction in the energy balance is proposed.
　The applicability of the calculation by the energy balance equation with diffraction term is investigated with the experiments on the transformation of directional random wave in model harbor and with the field survey results.
　The main conclusions are as follows:
1) When the directional distribution of wave energy in the numerical scheme becomes significant, numerical errors may be reduced by applying the diffraction term in energy balance equation.
2) The wave height observed in a small harbor of which the water depth varies in the range on its mouth to innermost part of harbor agrees with the estimated wave height in the newly developed energy balance equation.
3) The experimentally obtained wave height behind breakwaters agrees with the estimated wave height except of the points along the breakwater back side. The disagreement is indicated to the reflection wave break into back side. The good agreement demonstrates that the energy balance equation with diffraction term is applied to the prototype harbor design.