To provide a linear driving device for connecting a motor body and a ball screw shaft while miniaturizing the whole device.

This linear driving device 1 relatively linearly drives the ball screw shaft 11 to the motor body 1a by rotation of a shaft 6. A nut part 12 for a ball screw is arranged in a part corresponding to large diameter parts 51 and 61 among a screw shaft inserting hole 60 of the shaft 6. A ball screw nut 12 uses a long nut as the ball screw nut 12 by extending the large diameter part 61 including a tapered boundary part 611 in the shaft 6 up to a position near an armature core 3. The long nut can be used as the ball screw nut 12 by omitting a useless space by positioning a second radial bearing 4 by a level difference 64 of the shaft 6.