By using a simple model of gene conversion, the actual number of alleles contained in a multigene family was theoretically studied. It was shown that the Ewens' sampling theory is applicable to predict the actual number in a gene family of a genome. However, the actual number of the gene family forming the total population becomes larger or smaller than the predicted value by the sampling theory, depending upon the relative magnitude of the rates of two homogenization processes, i.e. intra-genome and in the population.