Background: The age-adjusted incidence rate of prostate cancer (PCa) has been reported to be lower among Asians than Western populations. A traditional Japanese meal, high in soybean products or isoflavones, may be associated with a decreased risk of PCa. Equol, which is converted from daidzein by human intestinal flora, is biologically more active than any other isoflavone aglycone. Materials and Methods: We reviewed not only recent epidemiological studies on association of isoflavones with PCa risk, but also recent research on human intestinal bacteria responsible for converting daidzein into equol. Studies were systematically searched from the database published within the last 5 years of from 2008-2012. Results: Five out of 6 articles showed significant association of isoflavones with a decreased risk of PCa, and two of them consistently showed that equol-producers carry a significantly reduced risk of PCa. Furthermore, 5 human intestinal bacteria that can convert daidzein into equol were identified in the last 5 years. Conclusions: If equol can reduce risk of PCa, a possible strategy for reducing the risk of PCa may be to increase the proportion of equol-producers by changing the intestinal flora to carrying an equol-producing bacterium with dietary alteration or probiotic technology.