The investment ratio between sons and daughters (sex allocation) is a serious concern for parents because it influences strongly the number of their future grands. Although it is 1 : 1 in most organisms, Neotermes koshunensis (Shiraki) [Kalotermitidae] is a unique termite which shows the numerical male-biased sex allocation. The ultimate and proximate mechanisms underlying this phenomenon remain unclear. In order to answer to the proximate mechanisms, we developed microsatellite genetic markers, some of which are estimated to link with the sex chromosome. And then, we investigated the sex ratio of the stage of eggs in this termite using this sex-linked marker. The results clearly proved that the sex ratio in eggs was biased toward males, which proves that this termite has the male-biased primary sex ratio. Future study will require the elucidation of the fertilization mechanism which favors toward males. This research is the first report of the primary sex ratio in termites and provides the important information for the understanding of the mechanism of the posterity adjustment in the organism.