A granular medium can be regarded as an equivalent uniform medium, of which the attenuation coefficient and the acoustic speed are obtained from the author's theory published elsewhere. The equivalent static density can be given by the mixed rule. According to the author's sound propagation theory in the granular medium, furthermore, the secondary waves are plane waves as well when the incidental sound wave is plane wave. In these cases, the three-dimensional equations can be simplified into a one-dimensional equation, and an expression for the viscosity coefficient in concentrated granular medium can be obtained. The theoretical results show that the viscosity coefficient depends on not only the volume fraction of the grains, but also sound frequency. On the other hand, the theory given in this paper can be used in the realistic cases, where less restrictions will be imposed on the applications than the Einstein's theory.