In algae, socially sexual induction through cell-to-cell interactions may be advantageous when probability of finding a mating partner is unpredictable. Here, we tested a hypothesis that surrounding potential mating partners induce sexual reproduction in the centric diatom Cyclotella meneghiniana. The diatom life cycle consists of a long vegetative phase and facultative sexual reproduction phase. Sexual reproduction is a critical step because it allows the cells to restore the large cell size; the cell diameter diminishes through mitosis. It is only when cells become smaller than a critical size that sexual reproduction can be induced, by environmental cue. In the centric diatoms, both sperms and eggs are generated from a single clone. Mixing small vegetative cells (sexualized with male-biased sex ratio by increasing salinity) with large vegetative cells (not sexualized), we demonstrated that sexual differentiation started in the large vegetative cells. Although the large cells did not sexualize through increasing salinity alone, differentiation into an egg was induced by the surrounding small cells. This flexibility may realize the effective cell size restoration and maximize their reproductive success in the fluctuating environment where the sex of their encountering cells in future is unpredictable.