PURPOSE: To reduce driving power and to reduce miniaturize the device by disposing ball splines on the sliding surfaces of an inside cylinder member and outside cylinder member of a supporting body having a cylinder mechanism of a single cylinder and transmitting the driving power of an actuator with a ball screw mechanism.

CONSTITUTION: This fixing device has the supporting body consisting of the inside cylinder member 8 and outside cylinder member 9 which are relatively freely movable along the longitudinal direction of a bone and is so constituted that pin holding members carrying the pins to be respectively inserted into the bones divided by a bone fracture are mounted at the inside cylinder member 8 and the outside cylinder member 9. Plural rows of the spline grooves 17a and 17b are formed to bace with each other along the moving direction (C-axis) on the sliding surfaces of the respective members 8, 9 and plural steel balls 16 are interposed therebetween. A motor 14 is housed in the inside cylinder member 8 and the screw shaft 13 is rotated via a speed reduction gear 15 by the operation thereof. The outside cylinder member 9 is moved along the C-axis by means of a ball screw mechanism 10 screwed thereto.