A ball screw unit has a plurality of partition-rooms each of them accommodating a spacer ball interposed between two load carrying balls. The partition room is formed into two oblique frustum shaped recesses each of them extends a flared mouth tilted downwards to the longitudinal axis of the ball screw unit. The partition room is made of elastic polymer material to facilitate pushing the spacer ball into an oblate ellipsoidal hole opened on the partition room, and the hole is also communicated with the two recesses. With this structure, the spacer balls can be held stably in the partition rooms without the fear of escaping or occurrence of disorganized ball squeezing during operation of the ball screw unit. Besides, the dimension of spacer ball may be made as small as possible so as to increase the number of load carrying balls thereby resulting in improvement of load carrying capability of the structure.