To provide a method and a device for estimating the quantity of muscles precisely and easily as much as possible by reducing the influence of the quantity of extracellular fluid which may be a cause of error in the estimation of quantity of muscles.

In the method of estimating the quantity of muscles, the resistance of the intracellular fluid of a subject is measured, and the quantity of muscles is estimated based on the resistance of the intracellular fluid. Accordingly, the quantity of muscles can be precisely estimated as the influence of the quantity of intracellular fluid, which tends to vary in the daily life, is reduced. The device for estimating the quantity of muscles comprises an intracellular fluid resistance measuring means for measuring the resistance of the extracellular fluid of the subject and a muscle quantity estimating means for estimating the quantity of muscles of the subject based on the measured resistance of the intracellular fluid. Accordingly, the device can precisely estimate the quantity of muscles as the influence of the quantity of the intracellular fluid, which tends to vary in the daily life, is reduced.