To investigate the switching system between aggregation and segregation in ants, we analyzed behaviors of ants in an enclosed chamber with a small size of population in detail. The following results were obtained: (i) generating aggregation needs a core individual, and (ii) an ant contacts with others continuously for a long duration time facilitates aggregation, (iii) the contact behavior itself facilitates segregation. At the lower population condition, the probability of participates in aggregation smaller than that of separation. However, as the population of aggregation increases, the probability of participates sigmoidally increases to exceed that of separation. This nonlinear change in the probability reflects the switching system between aggregation and segregation.