It is important to estimate an influence of permeable breakwaters in a numerical simulation for multidirectional random waves. In this study, a proposed method for generating transmission waves has been applied to the time-dependent mild slope equation for random waves. Transmission waves was generated by the wave damping method. The relationship between damping coefficients and transmission coefficients has been investigated by numerical simulations for uni-directional irregular waves. The validity of the method has been verified through the hydraulic model test for multi-directional random waves.