This study aimed to elucidate factors affecting the mental health status of older primary caregivers. Participants comprised 81 pairs of home care recipients aged ≥65 years and primary caregivers aged ≥65 years who were caring for the recipients. We used an individual interview method, which covered basic attributes, activities of daily living by Barthel Index, mental health status by Center for Epidemiologic Studies Depression Scale (CES-D), sense of coherence (SOC), and sense of care burden by Japanese version of the Zarit Burden Interview (J-ZBI). The score for the comprehensibility subscale of the SOC was significantly higher for the old-old caregivers compared with the young-old caregivers (P < .01). A significant positive correlation between the number of years of caregiving and the score for the SOC meaningfulness subscale was seen for the young-old caregivers (P < .05). For the old-old caregivers alone, negative correlations were seen between the CES-D score and the scores for all the SOC items. Multiple linear regression analysis using CES-D as the dependent variable showed a significant positive relationship to J-ZBI in all caregivers (P < .01). In contrast, a significant negative relationship was seen with meaningfulness, an SOC subscale only for the old-old caregivers. Meaningfulness as well as J-ZBI was extracted as a factor affecting the mental health status of the old-old caregivers, suggesting that higher SOC relates to lower stress levels with a remarkable decline in physical condition. Meaningfulness, an SOC subscale, is an important factor for improving the mental health of old-old caregivers.