In a deboning process, a knee joint length is measured in a work loading section of a first process station, and incision making of a thigh meat portion is performed in a second process station based on the measured knee joint length. At the same time, the entire length of a work is measured, and the knee joint length is calculated from previously collected data based on the measured entire length of the work. Positioning of a cutting blade and a meat separator is performed based on these values in subsequent process stations, and cutting of a tendon adhering to a knee joint and tearing of thigh meat are performed. A thigh bone is separated from a lower thigh in a further process station.