To smoothly and continuously travel/drive a conveying truck regardless of existence of a gap part between screw shafts for driving the conveying truck.

A plurality of screw shafts 1B, 1C are concentrically arranged in series at a distance from the gap part 13, and on the conveying truck 2, a position fixing driven roller 6 fitted between helical blades 1a, 1b at two front and rear positions in an axial direction of the screw shafts 1B, 1C, a movable driven roller 5 switchable to an application position and a non-application position relative to the screw shafts 1B, 1C and a retaining means 20 for retaining the movable driven roller 5 at the non-application position are provided. On the gap part 13 between the screw shafts, a switching means 23 for temporarily switching the movable driven roller 5 from the non-application position to the application position is also provided. While the position-fixing driven roller 6 is transferred from one side to the other side of the gap part 13, the movable driven roller 5 temporarily switched to the application position is fitted to the screw shafts 1B, 1C in an upstream side or a downstream side of the gap part 13.