The accumulation of the thermal noise can be considered to be one of the most important parameters for the effective design of a multirelay transmission system. A theoretical model for prediction of the distribution of this parameter has recently been proposed. The method has assumed the log-normal form for the unconditional point rainfall rate distribution. In the present work an extension of the previous method is proposed, to include the case of the gamma distribution which is more applicable to Japan and other regions with similar climatic conditions. Numerical results are compared with experimental data taken from a 20 GHz multirelay system located in Japan.