Understanding the adaptive significance of sex remains a fundamental issue in evolutionary biology. Asexual organisms are suitable materials to address this challenge. Furthermore, genetic uniformity in insect societies provides us a unique opportunity to explore the superorganismal adaptation. An East Asian ant, Monomorium triviale, is presumed to be a asexual species, but lacks direct evidence of asexual reproduction. Here, we provide solid evidence for asexuality of M. triviale with theoretical explanation of their social structure. Our field investigation showed the absence of males and inseminated queens. We confirmed worker production by virgin queens in laboratory, while no male was produced. In addition, genotyping indicated that the mother queens and daughter workers shared the identical genotype. Thus, our results strongly supported complete asexuality of this species. Field data also suggested that their nests were usually headed by multiple queens. We designed a mathematical model to estimate the optimal number of queens on the basis of inclusive fitness theory. Our model explains the presence of multiple queens in their nests under the degree of relatedness between queens we estimated. Taken together, our study serves as a basis for further investigation as a model system of epigenetic regulation mechanism underlying caste dimorphism.