In Japan, care-giving sons are noted for their high rates of abuse of care recipients. This study revealed, for the first time, the difficulties unique to sons caring for their mothers. All sons used long-term care insurance services. However, they provided care that could not be covered by such public services while harbouring psychological distress. In this study, we interviewed 13 sons living with their older mothers as the primary carer and conducted a thematic analysis focusing on their psychological distress. As part of our study findings, we have identified nine themes during analysis: uncomfortable feelings that cannot be mitigated as a son; feeling of futility in being unrecognised as a carer; fear of losing something important; a strong sense of responsibility for one's mother's life; disappointment in being unable to obtain understanding and co-operation from one's siblings in providing care; irritation and anger towards one's wayward mother; guilt about treatment of one's mother; sense of exhaustion due to demanding care; and experiencing hopelessness and despair. Thus, it is important to understand what psychological distress carers experience and to consider possible approaches for their support. The study findings provide suggestions for long-term care support in regions, such as East Asia, where the numbers of son carers are increasing inevitably owing to declining birth rates and ageing population, as in Japan.