An opening/closing body driving device for driving an opening/closing body is provided. The opening/closing body driving device includes: a motor, having a rotation shaft; a screw shaft, being rotated by the rotation shaft; a moving nut, being screwed to the screw shaft and moved in an axial direction of the screw shaft; and a connection part, being provided at the moving nut, wherein an arm for opening and closing the opening/closing body is connected to the connection part. The rotation shaft and the screw shaft are respectively provided on the same axis, and the connection part is provided at a radially outer side of the screw shaft.