PURPOSE: To simplify the structure and facilitate the assembling by providing positionally fixed pulleys on both ends of an extension frame, winding a belt thereon, fixing the extension frame in this state to the end part of a transfer base, and providing a power transmitting tension mechanism between the driven shaft of the extension-side pulley of the extension frame and a driving shaft on the side part of the transfer base.

CONSTITUTION: An extension frame 10 is mounted on the start end part 3 of a transfer base 1. An extension-side front pulley 11 is mounted on the front end of the extension frame, an extension-side rear pulley 12 is mounted on the rear end of the extension frame 10, and a belt 13 is laid over the extension- side front pulley 11 and the extension-side rear pulley 12. The extension frame 10 is fixed 17 by metal fittings 15, 16 so that it can be housed in the upper part at transfer and carrying, and horizontally extended only at use. When it is horizontally extended, the end surface 18 of the start end part 3 is butted to the end surface 19 of the extension frame 10 to keep the horizontality. A power transmitting mechanism 30 is provided between the start end part 3 of the transfer base l and the extension-side front pulley 11 of the extension frame 10.