PURPOSE: In a device where L-shaped cover frame bodies are fixed to two rotary shafts to be supported rotatably between supports provided at opposite ends of a load table, to improve the operationability by energizing each cover frame body in the opening direction through a torsion bar.

CONSTITUTION: Two rotary shafts 4, 4' are arranged in parallel with interval between C-shaped supports 3, 3' secured at the opposite ends of load table 2 of truck 1 while suspended rotatably in upper center of supports 3, 3' through bearings 5, 5'. Then L-shaped cover frame members 6, 6' made of pipe are fixed to each shafts 4, 4' to stretch a cover cloth over said frame bodies 6, 6'. While a plurality of torsion bars 7, 7' for energizing each cover frame body 6, 6' in the opening direction are fixed nearer to the center than each shaft 4, 4'. In other word, one end of torsion bar 7 is secured to a bracket 10 integral with a hollow pipe 8 penetrating through the shaft 4 while the other end is journaled by a bearing provided to the support 3 through a bracket.