In ant societies, conflicts over sex allocation between queens and workers can be predicted. To clarify the conflicts and the biasing mechanisms of the sex investment ratio, we observed the sex investment pattern in the monogynous and monandrous ant, Aphaenogaster smythiesi japonica Forel, the colonies of which are headed by single, once-mated queens. As a result of collecting 49 colonies over 3 years, the population sex investment ratio was about 1.5:1 females:males, and it was intermediate between the queen and worker optimum. However, the ratios at colony level largely varied within the population. The variation was strongly dependent on the colony size and larger colonies produced more new queens. This appears to be closely linked to the resource availability in each colony. Furthermore, the control mechanism of the sex investment ratio was investigated by comparing the primary sex ratio (the numerical proportion of haploid eggs to all eggs laid by queens) and the secondary sex ratio (the numerical proportion of males to all new adults). There was no difference between the primary and secondary sex ratios. Additionally, colony size, which strongly influenced the colony sex investment ratio, was correlated with neither primary nor secondary sex ratios, but with the numerical proportion of new queens to all new females. This suggests that A. s. japonica workers partly control sex allocation by manipulating the fate of the female caste, but not by eliminating male brood, according to the resource availability in their colonies.