The muscle cross-sectional area in the thigh decreases with aging, which lowers the capacity to carry out activities of daily living (ADL). In the present study, we tried to make a standard value for evaluating the muscle area of the thigh, considering sex, age and body size. Muscle cross-sectional areas of the thigh in 606 Japanese adults (257 males and 349 females) were measured using an ultrasound evaluation system. Because height and weight were related to age and muscle area, we proposed measured muscle area (cm2) × height (m)/weight (kg) as an index for evaluating muscle area. In both sexes, this index decreased with aging, and especially in females, the reduction was accelerated beyond 50 years old. These standard values should be useful for middle-aged and elderly people to prevent them from exhibiting a decrease in capacity to carry out ADL and becoming bedridden.