Cohabitation status––especially living alone––is another factor that potentially affects mental illness in older adults. In Japan in 1980, 4.3% of men and 11.2% of women lived alone, increasing to 11.1% of men and 20.3% of women by 2010 [12]. A recent Japanese study has suggested that living alone is associated with depression [13, 14]. The research suggests that although eating alone may increase the risk of psychological illness in older Japanese adults, these risks may be modified by cohabitation status. Although men and women have different social and domestic roles, to our knowledge, no studies have examined whether gender differences influence the association between eating alone and psychological illness. Therefore, we used data from a large-scale, population-based, longitudinal study to examine the effect of eating alone on depression among older Japanese women and men by cohabitation status.