To evaluate the efficiency of herbivore exclusion by the ant, Lasius japonicus Santschi on the vetch, Vicia angustifolia L. with extrafloral nectaries, we analyzed ant foraging behavior and ant aggressiveness against herbivores and discuss the mechanism of herbivore exclusion by ants in relation to the number of ants visiting a plant. In a field census, the number of alfalfa weevil Hypera postica larvae, which is the main herbivore on V. angustifolia, decreased with the number of L. japonicus workers. The encounter rate between workers of L. japonicus and herbivores increased as more workers visited the plant. The aggressiveness of an individual worker against herbivores also increased as more workers visited the plant. Consequently, there was a negative correlation between the number of workers on a plant and the residence time of an herbivore on a plant. Therefore, we expect that the efficiency of herbivore exclusion by ants increases rapidly when the number of ants on a plant attains a certain level.