To provide a mobile scaffolding device enabling the efficient execution of painting work for a main-truss upper chord of a bridge structure.

Cages 6 and 6 for scaffolding, which are movable downward, are annexed to both side surfaces of a working truck 1 traveling for movement on the main-truss upper chord A. The painting work for both the side surfaces and top surface of the upper chord A is carried out with the movement of the working truck 1 in a state wherein the cages 6 and 6 are made to descend by a hoisting and lowering means so as to face both the side surfaces of the upper chord A. Additionally, a scaffolding board 19 is placed between floor surfaces of the cages 6 and 6 on both sides in order to carry out the painting work for an under surface of the upper chord A. During arrival at a panel point part between the upper chord A and a main lateral truss, the cages 6 and 6 are made to ascend so that the panel point part can be gotten over by virtue of the movement of the working truck 1, and made to descend again for the execution of the painting work.