Beta glucan was purified from Phellinus baumii and tested for its effect on the group performance of ants, a principal model of eusociality and of superorganisms. Japanese carpenter ants (Camponotus japonicus) were reared on diets containing beta glucans to characterize their effects on eusociality. Culturing structures for ants were assembled by implanting autoclaved soil into polyethylene terephthalate bottles. Three different eusocial strength indices were used to study the effect of beta glucan extract (BGE) on eusocial activities—the number of residence chambers, cooperative defense index (CDI), and group size and composition. Control, low- BGE, and high-BGE diets were prepared with the following three levels of BGE supplements in sucrose powder: 0, 20, and 50% (g/g). More residence chambers were observed in the BGE-fed groups than in the control. The CDI against a foreign queen ant was calculated according to the time taken to subdue the foreign queen. The high-BGE group took less time to complete their defense formation than the other groups. Differences were evident between control and BGE-fed groups in the total numbers of ants and eggs. The BGE-fed groups showed a significant increase in both the number of workers and the number of eggs. When fed with BGE, ants responded positively for all three eusocial strength indices. These results show that BGE exerts beneficial effects on the eusociality of ants as a superorganism.