one side wall portion 29b being secured to an inner side wall of the box 2 on the other hand the upper roller 25 comprises a roller shaft portion 25a connected coaxially to the fore end of the long pin 19 and a roller portion 25b connected integrally with the roller shaft portion 25a the outer peripheral surface of the roller portion 25b is engaged with the upper surface of the bottom portion 29a of the support rail 29 in the state of face contact by such face contact of the roller portion 25b with the bottom portion 29a there is obtained a sufficient strength even when the upper roller 25 is integrally formed of a synthetic resin not a metal consequently there are attained reduction of the wear between the roller portion 25b and the bottom portion 29a improvement of durability decrease of noise and reduction of the cost the fore end of the chain 20 is loosely fitted movably on the chain fore end guide bar 30 allowing the chain 20 to move following the slat 5 further on both sides of the upper portion in the box 2 a pair of left and right chain driving sprockets 31 and 32 are journalled rotatably in the vicinity of the chain sprocket disengaging means 28 the sprockets 31 and 32 are integrally connected coaxially by a rotative shaft 33 as shown in fig8 to 10 the rotative shaft 33 comprises rotative shaft portions 33a and 33b of a small diameter attached to both side walls of the box 2 each rotatably through a bearing 34 and with the sprockets 31 and 32 integrally mounted thereon and a cylindrical shaft portion 33c of a large.