To investigate the prevalence of significant loss, potential complicated grief (CG), and its contributing factors, we conducted a nationwide random sampling survey of Japanese adults aged 18 or older (N = 1,343) using a self-rating Japanese-language version of the Complicated Grief Brief Screen. Among them, 37.0% experienced their most significant loss by expected non-violent death, 17.9% by unexpected non-violent death, and 5.5% by violent death. The mean length of time since the loss was 11.9 years (SD = 11.6). The percentage of individuals with potential CG (5 or higher score on the scale) was 2.5% among those who experienced significant loss. The individuals with potential CG showed lower mental health scores than those without. Through regression analysis, we found the significant effects of gender difference, time since the loss, and the interaction of the mode of death, gender of the bereaved, and the kinship relationship to the deceased on the CG score. Women who had lost a child by sudden or violent death showed significantly higher CG scores, but men did not. By comparison, those (particularly men) who had lost a partner by expected or sudden non-violent death showed significantly higher CG scores. The implications of the findings are discussed. (Contains 3 tables.)