A muscle activity amount determining device includes a shape obtainer that obtains a shape of a muscle of a user when the user is doing certain exercise a position identifying unit that identifies a position of the muscle of the user when the user is doing the exercise a determination circuit that refers to a determination reference that indicates a corresponding relationship between the position and shape of the muscle and an activity amount of the muscle, and determines the activity amount of the muscle using the position of the muscle identified by the position identifying unit and the shape of the muscle obtained by the shape obtainer and an outputter that outputs the activity amount, determined by the determination circuit.
