BACKGROUND: Suicide deaths are defined as deaths due to "intentional self-harm" in accordance with the International Statistical Classification of Diseases and Related Health Problems 10th Revision. However, "ill-defined and unknown causes of mortality," "accidents," and "event of undetermined intent" may also account for hidden suicides. This study compared the state of deaths due to suicide and ill-defined/undetermined causes across urban areas in Japan over the past 15 years using vital statistics. METHODS: We analyzed vital statistics data including information on all deaths identified under suicide and ill-defined/undetermined death category among Japanese citizens aged ≥10 years who lived in a government ordinance-designated city between 2008 and 2022. Standardized mortality ratios (SMRs) were calculated for each cause of death and compared among designated cities. RESULTS: Mortality rates due to "intentional self-harm" for those aged ≥65 years decreased during the study period, whereas mortality rates due to "ill-defined and unknown causes of mortality" for those aged ≥65 years sharply increased by nearly three times. Substantial differences in SMRs for "ill-defined and unknown causes of mortality" were observed among designated cities. There was no significant change in SMRs between the pre- and peri-COVID-19 pandemic periods in each designated city. CONCLUSIONS: In Japan, the state of deaths due to "ill-defined and unknown causes of mortality" substantially differed among designated cities. Attention must be paid to changes in the state of deaths due to ill-defined/undetermined causes as well as suicide deaths.