The early emergence (protandry) of males is common in a variety of insects. Most studies to date have explained protandry as an adaptive reproductive strategy. Sexual selection favors males that emerge before females because this maximizes the number of encounters with females. However, protandry may be a side effect of female-biased sexual size dimorphism (SSD) because larger females need a longer developmental time than males. Despite the plausibility of the adaptive protandry hypothesis, determining whether protandry is adaptive or incidental is difficult because size and developmental time are interdependent. Female-biased SSD is a common trend among insects and has been most frequently attributed to fecundity selection favoring large females. Reticulitermes speratus Kolbe alates showed significant SSD with female alates that were consistently heavier than male alates. A significant tendency was observed that SSD increased as body size increased among the colonies. A sexual difference in eclosion timing should be disadvantageous in this termite species because male and female alates perform swarming (nuptial flight) synchronously. Alates of an early emerging sex must use reserve nutrition while they wait for eclosion of the other sex in the colony. Nevertheless, significant protandry was found in R. speratus, and this tendency was detected even at the molt of the earlier nymphal stage. Therefore, the early emergence of males in this termite species is clearly incidental protandry due to the SSD. The occurrence of incidental protandry or protogyny as a side effect of sexual size dimorphism may be as common as adaptive protandry.