Eating alone while living with family members is a risk factor for mental health decline in old age. However, little is known as to why older adults choose to eat alone, even with family present. This study therefore aimed to explore reasons for older adults eating alone despite living with family members, using a qualitative approach. Fifteen people aged 65 years and older (11 men and 4 women) who were eating alone while living with family members were included in the study. These individuals were selected from the participants of the Kashiwa cohort study conducted in 2016. Individual interviews were conducted using an open-ended format. All interviews were recorded and transcribed. The data were further thematically analyzed using a qualitative software package, NVivo 11. We extracted six themes as reasons for eating alone and hypothesized interactions among these themes. The extracted themes were: "age-related changes," "solo-friendly environment," "family structure changes," "time lag for eating," "bad relationships with family members" and "routinization." To assess interactions, the themes were categorized as "background factors," "triggers," and "stabilizers." The aforementioned themes could lead to the development and sustained behavior of eating alone among older adults living with family members. As most themes describe conditions that are likely to remain static, it may not be realistic to encourage such individuals to begin eating with family members. The promotion of meals with neighbors or friends could be effective in alleviating the negative consequences of eating alone.