A rehabilitation instrument for ankle contracture prevention is proposed. Its uses a tendon drive system using a pneumatic balloon. This instrument is intended to be carried and installed easily at the time of use in a place like a medical institution. This instrument is for use by people such as those with flaccid hemiplegia. The ankle is moved when it is worn; ankle contracture can be prevented. The device comprises an actuator, a power transfer mechanism, and ankle foot orthosis. A tendon drive system using a pneumatic balloon was adopted as the actuator of this instrument. It consists of a tendon and a silicone tube. Both ends of the silicone tube are closed and the tube expands like a balloon with the supply of air, which distends the silicone tube and pulls the tendon. The system is compact and has high power. Furthermore, it is very lightweight because of its composition material and structure. This paper describes a study of characteristics of the tendon drive system using a pneumatic balloon, along with the composition and operation of a rehabilitation instrument for prevention of ankle contracture. Results of operation tests on the instrument are also presented.