Caregiving to older people with needs has been mainly dependent on informal care provision by female caregivers. Compared with the care burden gender gap, the within-gender gap in women's socioeconomic status (SES) has attracted less policy attention. We investigated the association between middle-aged women's SES and the likelihood of being a primary caregiver for elderly informal care, focusing on household income, women's marital status, work status, and educational background under the universal and public system of formal long-term care provision in Japan. We used repeated cross-sectional data from nationally representative household surveys conducted between 2010 and 2013 to obtain a sample of 2399 women aged between 40 and 60 years living in the same household as a care recipient. We conducted multiple logistic regression analysis to obtain odds ratios of being a primary caregiver in the household regressed on women's SES variables, adjusting for the characteristics of care recipients and household composition. The results showed that single women with lower education were likely to be primary caregivers when the care recipients had severe levels of care needs, whereas the association was null in the case of care recipients with milder conditions. The results indicated that women's low education and non-married status were related to a higher likelihood of becoming a primary caregiver of severely disabled elderly for reasons other than lower economic power. To emancipate socioeconomically vulnerable women from the care burden, a broader set of social, economic, and welfare policies are needed.