Maintaining good quality of life among elderly residents of nursing homes is becoming more important as their numbers increase with the ageing of society. Dry mouth, or xerostomia, is one condition that must be prevented, as it can lead to various serious diseases. However, older people are susceptible to dry mouth due to reduced lip closing force. In this study, a field survey of the indoor environment of nursing homes and oral dryness among nursing home residents was conducted in winter 2015. Twenty-seven facilities were classified as dry or moist based on measured indoor humidity. Ordered logistic regression analysis showed that the likelihood of dry mouth was significantly lower among patients residing in the moist group compared with the dry group.